remove plot_proc test function and test_plot_proc from test_utilities.py

[clmould]

test_plot_proc in test_utilities.py calls the test function within plot_proc. This is redundant since there is a test_plot_proc script and so this test function and test_utilities/test_plot_proc can be removed. This test also causes local issues depending upon order of tests (due to global scope), and so removing it will mean the tests pass as expected