Cleanup: Simplify arguments to deselect all nodes function

Take the node tree as an argument directly instead of retrieving it from
the editor struct. Then use the utility function in two more places.

source/blender/editors/space_node/node_edit.cc

@@ -2335,7 +2335,7 @@ static int node_clipboard_paste_exec(bContext *C, wmOperator *op)
   ED_preview_kill_jobs(CTX_wm_manager(C), CTX_data_main(C));
 
   /* deselect old nodes */
-  node_deselect_all(*snode);
+  node_deselect_all(*ntree);
 
   /* calculate "barycenter" for placing on mouse cursor */
   float2 center = {0.0f, 0.0f};