Revert "support of LLVM trunk after 201618 revision 'VirtualFileSystem'"

This reverts commit 7fcf408189.

module.cpp

@@ -1979,9 +1979,6 @@ void
 Module::execPreprocessor(const char *infilename, llvm::raw_string_ostream *ostream) const
 {
     clang::CompilerInstance inst;
-#if defined(LLVM_3_5)
-    inst.createVirtualFileSystem();
-#endif
     inst.createFileManager();
 
     llvm::raw_fd_ostream stderrRaw(2, false);