Compositor: Implement Levels node for new CPU compositor

Reference #125968.

source/blender/nodes/composite/nodes/node_composite_levels.cc

@@ -56,17 +56,6 @@ class LevelsOperation : public NodeOperation {
 
   void execute() override
   {
-    /* Not yet supported on CPU. */
-    if (!context().use_gpu()) {
-      for (const bNodeSocket *output : this->node()->output_sockets()) {
-        Result &output_result = get_result(output->identifier);
-        if (output_result.should_compute()) {
-          output_result.allocate_invalid();
-        }
-      }
-      return;
-    }
-
     if (get_input("Image").is_single_value()) {
       execute_single_value();
       return;