Remove no-op function pointer null checks, NFC

Null-checking functions which aren't marked weak_import is a no-op
(the compiler rewrites the check to 'true'), regardless of whether a
library providing its definition is weak-linked. If the deployment
target is greater than the minimum requirement, the availability markup
on APIs does not lower to weak_import.

Remove no-op null checks to clean up the code and silence warnings.

Differential Revision: https://reviews.llvm.org/D40812

llvm-svn: 319936