[clang][ifs] Dropping older experimental interface stub formats.

I've been working on a new tool, llvm-ifs, for merging interface stub files
generated by clang and I've iterated on my derivative format of TBE to a newer
format. llvm-ifs will only support the new format, so I am going to drop the
older experimental interface stubs formats in this commit to make things
simpler.

Differential Revision: https://reviews.llvm.org/D66573

llvm-svn: 369719