[AMDGPU] Remove unused MIMG tablegen variants

There are no AMDGPUSampleVariant versions for _G16, it is treated more like a
modifier for derivatives (_D) (also for intrinsics where it is overloaded type
instead of part of instrinsic name) so we ended up making more variants for
these instruction then we actually needed.

32-bit derivatives need 6 dwords at most, while 16-bit need 4 at most. Using
same AMDGPUSampleVariant for both, we ended up creating 2 extra variants per
instruction than were necessary.

In total this deletes 260 unused tablegen records.

Differential Revision: https://reviews.llvm.org/D131252