[libc++abi] Remove redundant null pointer check in operator delete

Similar to D52401. Normally operator delete is defined in libc++abi
(LIBCPP_DISABLE_NEW_DELETE_DEFINITIONS is off by default).

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.

free on MSDN:

    If memblock is NULL, the pointer is ignored and free immediately returns.

Reviewed By: #libc_abi, ldionne

Differential Revision: https://reviews.llvm.org/D93339