[mlir] Remove unnecessary assert for single region.

This was left over debugging.

mlir/lib/Transforms/Utils/RegionUtils.cpp

@@ -320,8 +320,6 @@ static LogicalResult deleteDeadness(MutableArrayRef<Region> regions,
 // This function returns success if any operations or arguments were deleted,
 // failure otherwise.
 static LogicalResult runRegionDCE(MutableArrayRef<Region> regions) {
-  assert(regions.size() == 1);
-
   LiveMap liveMap;
   do {
     liveMap.resetChanged();