[Frontend] Remove unused TemporaryFiles

Summary:
OnDiskData.TemporaryFiles is filled only by ASTUnit::addTemporaryFile, which is
dead. Also these files are used nowhere in the frontend nor in libclang.

Reviewers: bkramer, ilya-biryukov

Reviewed By: bkramer, ilya-biryukov

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D33270

llvm-svn: 303265