[libcxx] Remove the availability_markup LIT feature

It is now equivalent to the 'availability' LIT feature, so there's no
reason to keep both.

llvm-svn: 348653
Cr-Mirrored-From: sso://chromium.googlesource.com/_direct/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: 737be83d20cfa45f13b182edf13ab188ae345e5c
12 files changed