[clang-tidy][docs] Remove the entry about 'cppcoreguidelines-init-variables' from the release notes

The patch in http://reviews.llvm.org/D106431 landed in commit
4a097efe to the main branch.
However, this patch was also backported to upcoming release 13.0.0 in
commit 8dcdfc0de84f60b5b4af97ac5b357881af55bc6e, which makes this entry
in the release notes **NOT** a new thing for the purposes of 14.0.0.