Dedupe update of rcParams["backend"] in use() and in switch_backend()

[anntzer]

use() calls switch_backend() to actually perform the backend switching,
so we don't need to duplicate the setting of rcParams["backend"] and
rcParamsDefault["backend"] in use() as well.

Also tiny markup fix for rcdefaults().

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