Remove NO_EDITOR/NO_CUSTOM_LEVELS, disable editor on Steam Deck

This commit removes the `NO_EDITOR` and `NO_CUSTOM_LEVELS` defines,
which cleans up the code a lot, and they weren't really needed anyways.

This commit also disables the editor on the Steam Deck, and adds a
program argument to re-enable the editor, `-enable-editor`.

desktop_version/src/RenderFixed.cpp

@@ -131,7 +131,6 @@ void gamerenderfixed(void)
 
     map.updateroomnames();
 
-#if !defined(NO_CUSTOM_LEVELS) && !defined(NO_EDITOR)
     ed.old_return_message_timer = ed.return_message_timer;
     if (map.custommode && !map.custommodeforreal && ed.return_message_timer > 0)
     {
@@ -165,7 +164,6 @@ void gamerenderfixed(void)
             }
         }
     }
-#endif
 }
 
 void titlerenderfixed(void)