backend_wx: delete remaining lines for removal of printer support

This just finishes off the process; most of the removal was done
in #84ffb60.
Merge pull request #3839 from efiring/wx_printer_last_deletion

backend_wx: delete remaining lines for removal of printer support
Allow style.use() to accept RcParams instances.
Remove isinstance() calls in mpl.styles.use().
Add unit test for style.context() using a dict.
PEP 8.
Update docstring for style.use() and style.context().
Add more informative unit test names. Style cleanup.
Use table for docstring.
Reset style values on any error.
Fixing "line too long (80 > 79 characters)". :-/
Missing import for SkipTest.
Merge pull request #3795 from chebee7i/style-rcparams

ENH : RcParams/dict as input to matplotlib.style.use