[GlobalsModRef] Remove check for allocator calls

As the FIXME already indicates, I don't see why this code would be
necessary. If there's a call to an allocator function, that should
get treated just like any other function call -- usually it will be
a declaration and handled conservatively based on memory attributes
only. There should be no need to explicitly force it to be modref.
No test failures either, so I think this is just dead code.

Differential Revision: https://reviews.llvm.org/D127273