[Libomptarget][NFC] Fix unused variable warnings

Summary:
This patch fixes a few warnings that would show up while building.

Author: jhuber6

openmp/libomptarget/plugins-nextgen/amdgpu/src/rtl.cpp

@@ -1885,8 +1885,8 @@ struct AMDGPUDeviceTy : public GenericDeviceTy, AMDGenericDeviceTy {
     // Get the frequency of the steady clock. If the attribute is missing
     // assume running on an older libhsa and default to 0, omp_get_wtime
     // will be inaccurate but otherwise programs can still run.
-    if (auto Err = getDeviceAttrRaw(HSA_AMD_AGENT_INFO_TIMESTAMP_FREQUENCY,
-                                    ClockFrequency))
+    if (getDeviceAttrRaw(HSA_AMD_AGENT_INFO_TIMESTAMP_FREQUENCY,
+                         ClockFrequency) != HSA_STATUS_SUCCESS)
       ClockFrequency = 0;
 
     // Load the grid values dependending on the wavefront.

openmp/libomptarget/src/interface.cpp

@@ -495,7 +495,7 @@ EXTERN void __tgt_target_nowait_query(void **AsyncHandle) {
   if (QueryCounter.isAboveThreshold())
     AsyncInfo->SyncType = AsyncInfoTy::SyncTy::BLOCKING;
 
-  if (const int Rc = AsyncInfo->synchronize())
+  if (AsyncInfo->synchronize())
     FATAL_MESSAGE0(1, "Error while querying the async queue for completion.\n");
   // If there are device operations still pending, return immediately without
   // deallocating the handle and increase the current thread query count.