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/Entity.cpp

@@ -1287,7 +1287,6 @@ void entityclass::createentity(int xp, int yp, int t, int meta1, int meta2, int
     //Rule 6 is a crew member
 
     bool custom_gray;
-#if !defined(NO_CUSTOM_LEVELS)
     // Special case for gray Warp Zone tileset!
     if (map.custommode)
     {
@@ -1295,7 +1294,6 @@ void entityclass::createentity(int xp, int yp, int t, int meta1, int meta2, int
         custom_gray = room->tileset == 3 && room->tilecol == 6;
     }
     else
-#endif
     {
         custom_gray = false;
     }