[libcxx] Remove the availability_markup LIT feature

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

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@348653 91177308-0d34-0410-b5e6-96231b3b80d8
12 files changed