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/Editor.cpp

@@ -1,5 +1,3 @@
-#if !defined(NO_CUSTOM_LEVELS) && !defined(NO_EDITOR)
-
 #define ED_DEFINITION
 #include "Editor.h"
 
@@ -4161,5 +4159,3 @@ void editorclass::switch_warpdir(const bool reversed)
 
     graphics.backgrounddrawn = false;
 }
-
-#endif /* NO_CUSTOM_LEVELS and NO_EDITOR */