[AMDGPU] Remove unnecessary conditionality on atomic CSUB pseudo-ops (#69914)

The pseudo-ops for BUFFER_ATOMIC_CSUB and GLOBAL_ATOMIC_CSUB should not
be conditional on FeatureAtomicCSubNoRtnInsts, as that feature rightly
should only control whether or not the non-returning forms are available
for instruction selection, not whether they exist or not.

Change-Id: Icb2fcdcf6c7bc55e48b522d0eec8dd35637c768e