Allow disabling the explicit use of CUDA textures
This change implements fallback for the explicit CUDA texture loads
in the non-bonded kernels. This can be done by defining
DISABLE_CUDA_TEXTURES. When disabled texture objects/references
are not initialized either.
Also removed unnecessary extern declarations of texture references
in nbnxn_cuda_kernel_utils.cuh; this was only needed because texture
reference accesses were previously compiled unconditionally (and were
also generated in the nvcc host pass).
Change-Id: Id7cdd6f80da0abe6be5639e80bed6530c3ce25c0