build: remove --enable-lcov-branch-coverage

This supports lcov 2.x in the sense that we are no-longer hardcoding
version specific options, and instead will use the `LCOV_OPTS` variable
(which is the more correct/flexible thing to do in any case). It's also
quite likely that devs are already having to pass extra options to lcov
2.x, given it's more stringent in terms of coverage generation and error
checking. See this thread for an example:
https://github.com/linux-test-project/lcov/issues/238.

Added an example to the developer notes.

Tested on one machine (LCOV 2.0, gcc 13.2) with:
```bash
./autogen.sh
./configure --enable-lcov CXXFLAGS="-fprofile-update=prefer-atomic" LCOV_OPTS="--rc branch_coverage=1 --ignore-errors mismatch"
make
make cov
<snip>
Processing file src/netaddress.cpp
  lines=521 hit=480 functions=72 hit=72 branches=675 hit=499
Overall coverage rate:
  lines......: 81.8% (79362 of 97002 lines)
  functions......: 77.8% (10356 of 13310 functions)
  branches......: 49.6% (130628 of 263196 branches)
```

and another machine (LCOV 2.1, Clang 18.1.3) with:
```bash
./autogen.sh
./configure --enable-lcov CC=clang CXX=clang++ LCOV_OPTS="--rc branch_coverage=1 --ignore-errors mismatch,inconsistent"
make
make cov
<snip>
Processing file src/util/strencodings.cpp
  lines=315 hit=311 functions=38 hit=38 branches=425 hit=357
Overall coverage rate:
  source files: 622
  lines.......: 79.8% (70311 of 88132 lines)
  functions...: 78.1% (13968 of 17881 functions)
  branches....: 44.5% (157551 of 354317 branches)
Message summary:
  101 warning messages:
    count: 1
    inconsistent: 100
  3528 ignore messages:
    inconsistent: 3528
```

0 files changed, 0 insertions, 0 deletions