Don't cache the charmap and inverse charmap

mathtext creates Python dictionaries for the charmap and inverse charmap
for each font.  This turns out to be unnecessary:

1) freetype has an API to do a charmap lookup that is faster than a
Python dictionary

2) The inverse charmap isn't really necessary if we convert the
latex_to_bakoma to use unicode character points rather than glyph
indices.

This should have a large impact when #5241 is merged with larger fonts.