Rely on findfont autofallback-to-default in pdf/ps backends.

[anntzer]

findfont(...) will never return None unless fallback_to_default=False
is passed -- the default is True and causes afm searches to fall
back to Helvetica -- so the "filename is None" branch never occurs.
(The code in the pdf/ps backends came in in af9954d (2009) whereas
fallback_to_default only came in in 94120dc (2010).)

Noted while reviewing #15629.

PR Summary

PR Checklist

• Has Pytest style unit tests
• Code is Flake 8 compliant
• New features are documented, with examples if plot related
• Documentation is sphinx and numpydoc compliant
• Added an entry to doc/users/next_whats_new/ if major new feature (follow instructions in README.rst there)
• Documented in doc/api/api_changes.rst if API changed in a backward-incompatible way