Remove checkHeap from newSlab

Summary: Running a sandbox with dbgo is way slow now, and most of the time is
spent in checkHeap, called from newSlab.  This seems pretty expensive even for a
debug build.

Reviewed By: @edwinsmith

Differential Revision: D1748981

Signature: t1:1748981:1418937990:80315f28aed008ede610e48c894464dfda936424