clutter: Drop unused private functions/defines

The commit also stops defining certain functions in a private header
when they are only used in the same file.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3437>

clutter/clutter/clutter-input-focus.c

@@ -25,6 +25,14 @@
 
 typedef struct _ClutterInputFocusPrivate ClutterInputFocusPrivate;
 
+static void clutter_input_focus_delete_surrounding (ClutterInputFocus *focus,
+                                                    int                offset,
+                                                    guint              len);
+static void clutter_input_focus_set_preedit_text (ClutterInputFocus *focus,
+                                                  const gchar       *preedit,
+                                                  unsigned int       cursor,
+                                                  unsigned int       anchor);
+
 struct _ClutterInputFocusPrivate
 {
   ClutterInputMethod *im;