[NVPTX] Remove unsupported 'seq_cst' test (#156088)
The NVPTX backend does not currently support correctly lowering
`atomicrmw` with `seq_cst` of any size. Remove a test which erroneously
hit this case and add logic to appropriately error out when this
ordering is encountered, instead of emitting invalid PTX.
In the long term, we should use `fence.sc`, similar to other atomic
operations, to support this ordering.
