Remove grdimage, legend and plot3d xfail markers

[seisman]

Description of proposed changes

• The test_plot3d_matrix_color xfail on Windows should be addressed in the discussions in Update colorbar, inset, legend and logo baseline images for GMT 6.2.0 #1322.
• The legend and grdimages tests were marked as xfail because the baseline images should be updated due to upstream changes. However, now we have ~60 failures with the GMT dev branch (tracked in WIP: Update baseline images for the GMT 6.4 part 1 #1791), we should also remove the xfail markers for these tests.

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 wrapping a new module, open a 'Wrap new GMT module' issue and submit reasonably-sized PRs.
• If adding new functionality, add an example to docstrings or tutorials.

Slash Commands

You can write slash commands (/command) in the first line of a comment to perform
specific operations. Supported slash commands are:

• /format: automatically format and lint the code
• /test-gmt-dev: run full tests on the latest GMT development version