Merge ENABLE_THREADS and ENABLE_STD_THREADS markers

Both of these markers are used in the test suit for annotating when a
test needs multi threaded support. Previously they had slightly
different meening but they converged to the point where they are used
interchangably. This CL removes the ENABLE_STD_THREADS one to simplify
the test suite and avoid some confusion.

Differential revision: http://reviews.llvm.org/D15498

llvm-svn: 255641