Remove Fonts.get_used_characters

This is never called, is on an internal class, and the attribute it
accesses doesn't exist as it was removed in matplotlib#22204.

Author: QuLogic

lib/matplotlib/_mathtext.py

@@ -243,14 +243,6 @@ def get_underline_thickness(self, font, fontsize, dpi):
         """
         raise NotImplementedError()
 
-    def get_used_characters(self):
-        """
-        Get the set of characters that were used in the math
-        expression.  Used by backends that need to subset fonts so
-        they know which glyphs to include.
-        """
-        return self.used_characters
-
     def get_sized_alternatives_for_symbol(self, fontname, sym):
         """
         Override if your font provides multiple sizes of the same