Revert "Make skipIf support the not_in function."

Summary:
This reverts commit 251965377bdfb6227eea42c12a792c059e4e8a4b
as a test marked "skipIf(compiler='gcc')" runs when testing with GCC.

Reviewers: amccarth

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D14852

llvm-svn: 253631