remove redundant test_utilities/test_plot_proc and test function from…

[clmould]

… plot_proc

Description

Checklist

I confirm that I have completed the following checks:

• [ ] I have justified any large differences in the regression tests caused by this pull request in the comments.
• [ ] I have added new tests where appropriate for the changes I have made.
• [ ] If I have had to change any existing unit or integration tests, I have justified this change in the pull request comments.
• [ ] If I have made documentation changes, I have checked they render correctly.
• [ ] I have added documentation for my change, if appropriate.