Remove xcode-specific Config.h

Summary:
Now that the xcode project is removed, we no longer need/use the
hand-maintained Config.h file, as everything is configured through
cmake.

This patch deletes that file and reverts some of the changes from
r300372, which were made to support this use case.

Reviewers: sgraenitz, beanz

Subscribers: mgorny, lldb-commits

Differential Revision: https://reviews.llvm.org/D65862

llvm-svn: 368266