cpu: Remove SLC bit restraint for GPU tester (#552)

This reverts gem5#133, the temporary work-around for gem5#131, allowing
both SLC and GLC atomic requests to be made in the GPU tester.

The underlying issues behind gem5#131 have been resolved by gem5#367 and
gem5#397.

src/cpu/testers/gpu_ruby_test/gpu_wavefront.cc

@@ -189,7 +189,6 @@ GpuWavefront::issueAtomicOps()
                                              AtomicOpFunctorPtr(amo_op));
         req->setPaddr(address);
         req->setReqInstSeqNum(tester->getActionSeqNum());
-        req->setCacheCoherenceFlags(Request::SLC_BIT);
         // set protocol-specific flags
         setExtraRequestFlags(req);