[llvm-cov gcov] Drop support for clang<11's fake GCC 4.2 format

clang<11 intended to use a GCC 4.2 compatible format but that turned out
to make some mistakes. After instrumentation side format changes,
c8b082a3 kept supporting the legacy
format, originally intended for 1 or 2 major releases.

Remove an obsoleted comment as GCC 12 uses length==12 for
GCOV_TAG_FUNCTION in .gcda files.