Drop gtk_main_level

This api isn't useful without gtk_main, which is also
on the way out.

docs/reference/gtk/gtk4-sections.txt

@@ -4419,7 +4419,6 @@ gtk_get_locale_direction
 gtk_init
 gtk_init_check
 gtk_main
-gtk_main_level
 gtk_main_quit
 
 <SUBSECTION>

gtk/gtkmain.c

@@ -1124,20 +1124,6 @@ gtk_main_sync (void)
   _gtk_recent_manager_sync ();
 }
 
-/**
- * gtk_main_level:
- *
- * Asks for the current nesting level of the main loop.
- *
- * Returns: the nesting level of the current invocation
- *     of the main loop
- */
-guint
-gtk_main_level (void)
-{
-  return gtk_main_loop_level;
-}
-
 /**
  * gtk_main_quit:
  *

gtk/gtkmain.h

@@ -113,8 +113,6 @@ GtkTextDirection gtk_get_locale_direction (void);
 GDK_AVAILABLE_IN_ALL
 void       gtk_main                (void);
 GDK_AVAILABLE_IN_ALL
-guint      gtk_main_level          (void);
-GDK_AVAILABLE_IN_ALL
 void       gtk_main_quit           (void);
 
 GDK_AVAILABLE_IN_ALL