Remove redundant null pointer check in operator delete

Summary:
C89 4.10.3.2 The free function
C99 7.20.3.2 The free function
C11 7.22.3.3 The free function

    If ptr is a null pointer, no action shall occur.

_aligned_free on MSDN:

    If memblock is a NULL pointer, this function simply performs no actions.

Reviewers: EricWF, mclow.lists, khng300, hotpxl

Reviewed By: mclow.lists, khng300, hotpxl

Subscribers: lichray, llvm-commits, hotpxl, khng300, christof, ldionne, cfe-commits, libcxx-commits

Differential Revision: https://reviews.llvm.org/D52401

llvm-svn: 343503