ASTUnit: remove dead code in remapping files

ASTUnit contains code to remap files to other files on disk.  This code is not
used.  We only remap files to MemoryBuffers.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@201010 91177308-0d34-0410-b5e6-96231b3b80d8