remove the fontsize check in xarray.tests.test_plot.test_no_args, bec…

[guziy]

…ause it can be greater than 12 for high-resolution screens

• Closes Failing test, maybe it could be removed? #1584
• Tests passed
  Not sure why continuous integration checks are failing. Do not hink this is because of the change, since the master branch is also failing the continuous integration checks.
• Passes git diff upstream/master | flake8 --diff
• Fully documented, including whats-new.rst for all changes and api.rst for new API
  Change is too small for whats-new.rst or api.rst.

[shoyer]

Thanks!