Remove polylith/API mode (#2967)

This removes most of the code used for polylith/API mode.

This removes the `/api` internal endpoints entirely. 

Binary size change roughly 5%: 
```
51437560 Feb 13 10:15 dendrite-monolith-server # old
48759008 Feb 13 10:15 dendrite-monolith-server # new
```

4 files changed, 4 insertions, 26 deletions

diff --git a/clientapi/routing/auth_fallback_test.go b/clientapi/routing/auth_fallback_test.go
index 0d77f9a0..534581bd 100644
--- a/clientapi/routing/auth_fallback_test.go
+++ b/clientapi/routing/auth_fallback_test.go
@@ -22,7 +22,7 @@ func Test_AuthFallback(t *testing.T) {
 			for _, wantErr := range []bool{false, true} {
 				t.Run(fmt.Sprintf("useHCaptcha(%v) - recaptchaEnabled(%v) - wantErr(%v)", useHCaptcha, recaptchaEnabled, wantErr), func(t *testing.T) {
 					// Set the defaults for each test
-					base.Cfg.ClientAPI.Defaults(config.DefaultOpts{Generate: true, Monolithic: true})
+					base.Cfg.ClientAPI.Defaults(config.DefaultOpts{Generate: true, SingleDatabase: true})
 					base.Cfg.ClientAPI.RecaptchaEnabled = recaptchaEnabled
 					base.Cfg.ClientAPI.RecaptchaPublicKey = "pub"
 					base.Cfg.ClientAPI.RecaptchaPrivateKey = "priv"
@@ -33,7 +33,7 @@ func Test_AuthFallback(t *testing.T) {
 						base.Cfg.ClientAPI.RecaptchaSitekeyClass = "h-captcha"
 					}
 					cfgErrs := &config.ConfigErrors{}
-					base.Cfg.ClientAPI.Verify(cfgErrs, true)
+					base.Cfg.ClientAPI.Verify(cfgErrs)
 					if len(*cfgErrs) > 0 {
 						t.Fatalf("(hCaptcha=%v) unexpected config errors: %s", useHCaptcha, cfgErrs.Error())
 					}
diff --git a/clientapi/routing/leaveroom.go b/clientapi/routing/leaveroom.go
index 86414afc..a7166185 100644
--- a/clientapi/routing/leaveroom.go
+++ b/clientapi/routing/leaveroom.go
@@ -18,7 +18,6 @@ import (
 	"net/http"
 
 	"github.com/matrix-org/dendrite/clientapi/jsonerror"
-	"github.com/matrix-org/dendrite/internal/httputil"
 	roomserverAPI "github.com/matrix-org/dendrite/roomserver/api"
 	"github.com/matrix-org/dendrite/userapi/api"
 	"github.com/matrix-org/util"
@@ -45,15 +44,6 @@ func LeaveRoomByID(
 				JSON: jsonerror.LeaveServerNoticeError(),
 			}
 		}
-		switch e := err.(type) {
-		case httputil.InternalAPIError:
-			if e.Message == jsonerror.LeaveServerNoticeError().Error() {
-				return util.JSONResponse{
-					Code: http.StatusForbidden,
-					JSON: jsonerror.LeaveServerNoticeError(),
-				}
-			}
-		}
 		return util.JSONResponse{
 			Code: http.StatusBadRequest,
 			JSON: jsonerror.Unknown(err.Error()),
diff --git a/clientapi/routing/register.go b/clientapi/routing/register.go
index be2b192b..ff6a0900 100644
--- a/clientapi/routing/register.go
+++ b/clientapi/routing/register.go
@@ -31,8 +31,6 @@ import (
 	"time"
 
 	"github.com/matrix-org/dendrite/internal"
-	internalHTTPUtil "github.com/matrix-org/dendrite/internal/httputil"
-	"github.com/matrix-org/dendrite/internal/sqlutil"
 	"github.com/tidwall/gjson"
 
 	"github.com/matrix-org/dendrite/internal/eventutil"
@@ -861,16 +859,6 @@ func completeRegistration(
 				JSON: jsonerror.UserInUse("Desired user ID is already taken."),
 			}
 		}
-		switch e := err.(type) {
-		case internalHTTPUtil.InternalAPIError:
-			conflictErr := &userapi.ErrorConflict{Message: sqlutil.ErrUserExists.Error()}
-			if e.Message == conflictErr.Error() {
-				return util.JSONResponse{
-					Code: http.StatusBadRequest,
-					JSON: jsonerror.UserInUse("Desired user ID is already taken."),
-				}
-			}
-		}
 		return util.JSONResponse{
 			Code: http.StatusInternalServerError,
 			JSON: jsonerror.Unknown("failed to create account: " + err.Error()),
diff --git a/clientapi/routing/register_test.go b/clientapi/routing/register_test.go
index bccc1b79..670c392b 100644
--- a/clientapi/routing/register_test.go
+++ b/clientapi/routing/register_test.go
@@ -201,8 +201,8 @@ func TestValidationOfApplicationServices(t *testing.T) {
 	// Set up a config
 	fakeConfig := &config.Dendrite{}
 	fakeConfig.Defaults(config.DefaultOpts{
-		Generate:   true,
-		Monolithic: true,
+		Generate:       true,
+		SingleDatabase: true,
 	})
 	fakeConfig.Global.ServerName = "localhost"
 	fakeConfig.ClientAPI.Derived.ApplicationServices = []config.ApplicationService{fakeApplicationService}