Remove darwin_fat.mk.

Summary:
Its seems to be replaced by clang_darwin.mk in the Makefile-based
build, and is only referenced in unittest scripts, which are
broken for a long time now.

Test Plan: n/a

Reviewers: bob.wilson

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D6574

llvm-svn: 224946