Revert plotting across dateline workarounds

[weiji14]

Description of proposed changes

Remove some workarounds that were added because plotting grids across the dateline would crash, not needed since we've bumped the minimum required GMT version to 6.1.1 at #577. These are cherrypicked commits from #560 to keep diff small. Specifically, this PR will:

• Remove the runfirst workaround added in Properly allow for either pixel or gridline registered grids #476, so there won't be a two-step make test in our Makefile anymore
• Remove grdtrack gallery example workaround (due to a segfault crash) added in Fix the grdtrack example to avoid crashes on macOS #531.
• Unregister setup.cfg pytest markers runfirst, and also 'mpl_image_compare` added in Register mpl_image_compare marker to remove PytestUnknownMarkWarning #323 (since the issue has been fixed upstream at Register mpl_image_compare marker matplotlib/pytest-mpl#83)

Fixes #

Reminders

• Run make format and make check to make sure the code follows the style guide.
• Add tests for new features or tests that would have caught the bug that you're fixing.
• Add new public functions/methods/classes to doc/api/index.rst.
• Write detailed docstrings for all functions/methods.
• If adding new functionality, add an example to docstrings or tutorials.

[seisman]

Looks good.