[NVPTX] Remove image/sampler special case in call lowering

I suspect that this is dead code. There is no test coverage for
this special case, and the struct type names this checks against
don't seem to match what OpenCL actually generates (which would be
%opencl.sampler_t rather than %struct._sampler_t for example).

Motivation for this change is that this code is incompatible with
opaque pointers -- simply deleting it is the simplest way of
making it compatible :)

Differential Revision: https://reviews.llvm.org/D119229