[clang][NFC] Remove unused private variable 'CI' in CrossTranslationUnit.h

It seems because of the recent refactorings this variable has become unused
and now we get this warning in the build logs:

In file included from llvm/clang/lib/CrossTU/CrossTranslationUnit.cpp:12:
llvm/clang/include/clang/CrossTU/CrossTranslationUnit.h:200:21: warning: private field 'CI' is not used [-Wunused-private-field]
  CompilerInstance &CI;
                    ^

I'll remove them for now to get the builds back to green.

llvm-svn: 367840