[libc++/abi] Re-remove unnecessary null pointer checks from operator delete

In 7cd67904, we removed the unnecessary nullptr checks from the libc++abi
definition of operator delete, but we forgot to update the definition in
libc++ (damn code duplication!). Then, in d4a1e03c, I synced the
definitions across libc++ and libc++abi, but I did it the wrong way around.
I re-added the if() checks to libc++abi instead of removing them from libc++.

In ef74f0fd, we re-removed the if() check from operator delete, but
only in libc++abi. This patch corrects this mess and removes it
consistently in libc++ and libc++abi.

Differential Revision: https://reviews.llvm.org/D93473