BUG: _plot_mri_contours makes too many figures

[larsoner]

@hoechenberger it looks like https://github.com/mne-tools/mne-python/pull/9754/files#r719376494 was never actually addressed in #9754 unless I'm missing it somehow in the diff?

[larsoner]

... also it seems weird to have moved the fig creation out of the conditional, just to close the figure:

https://github.com/mne-tools/mne-python/pull/9754/files#diff-29e49a197d8f4e70c13388d2374c60cb9a2811aeb0ccfaf67e39ec82717456dbR392-R397

It seems like we should only create this figure if it's needed, rather than always creating it and closing it if it's not needed. EDIT: though maybe I'm missing some reason it needs to be created, since it was created in both cases before, too...?

[hoechenberger]

I think I simply forgot about this change request in the end. I will take a look shortly