p2p: Remove BIP61 reject messages

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+P2P and network changes
+-----------------------
+
+#### Removal of reject network messages from Bitcoin Core (BIP61)
+
+The command line option to enable BIP61 (`-enablebip61`) has been removed.
+
+This feature has been disabled by default since Bitcoin Core version 0.18.0.
+Nodes on the network can not generally be trusted to send valid ("reject")
+messages, so this should only ever be used when connected to a trusted node.
+Please use the recommended alternatives if you rely on this deprecated feature:
+
+* Testing or debugging of implementations of the Bitcoin P2P network protocol
+  should be done by inspecting the log messages that are produced by a recent
+  version of Bitcoin Core. Bitcoin Core logs debug messages
+  (`-debug=<category>`) to a stream (`-printtoconsole`) or to a file
+  (`-debuglogfile=<debug.log>`).
+
+* Testing the validity of a block can be achieved by specific RPCs:
+  - `submitblock`
+  - `getblocktemplate` with `'mode'` set to `'proposal'` for blocks with
+    potentially invalid POW
+
+* Testing the validity of a transaction can be achieved by specific RPCs:
+  - `sendrawtransaction`
+  - `testmempoolaccept`
+
+* Wallets should not use the absence of "reject" messages to indicate a
+  transaction has propagated the network, nor should wallets use "reject"
+  messages to set transaction fees. Wallets should rather use fee estimation
+  to determine transaction fees and set replace-by-fee if desired. Thus, they
+  could wait until the transaction has confirmed (taking into account the fee
+  target they set (compare the RPC `estimatesmartfee`)) or listen for the
+  transaction announcement by other network peers to check for propagation.