Remove check_figures_equal

Co-authored-by: Wei Ji <[email protected]>

Author: seisman

pygmt/tests/test_inset.py

@@ -3,7 +3,6 @@
 """
 import pytest
 from pygmt import Figure
-from pygmt.helpers.testing import check_figures_equal
 
 
 @pytest.mark.mpl_image_compare