Remove incorrect assert in button group code

Even after the last commit to fix this assert, it still fails in the
case where a button was added before there was a button group
added to the block. Another fix for this would be to always create
a button group in UI_block_begin, but this assert has no particular
purpose, so it's simpler to just remove it.

source/blender/editors/interface/interface_button_group.c

@@ -86,9 +86,6 @@ void ui_button_group_replace_but_ptr(uiBlock *block, const void *old_but_ptr, ui
       }
     }
   }
-
-  /* The button should be in a group, otherwise there are no button groups at all. */
-  BLI_assert(BLI_listbase_is_empty(&block->button_groups));
 }
 
 /** \} */