[libc++][doc] Removes LLVM-17 release notes. (#78062)

The old notes are kept to make it easier to backport changes to the
release branch. There are no LLVM-17 releases expected and this
documentation should not be available in the LLVM-18 release.

Note after branching LLVM-18 both LLVM-18 and LLVM-19 release notes
should be available.