Remove unused `import sunpy.io.fits`

[TrestanSimon]

This PR removes the line https://github.com/dstansby/pfsspy/blob/4303bfbf62b37f11dedadfaff5be64e78c720577/examples/using_pfsspy/plot_aia_overplotting.py#L13 as sunpy.io.fits is both deprecated and no longer used to read magnetic field data.

The Read the Docs build of #362 is failing due to the former:

Unexpected failing examples:
/home/docs/checkouts/readthedocs.org/user_builds/pfsspy/checkouts/362/examples/using_pfsspy/plot_aia_overplotting.py failed leaving traceback:
Traceback (most recent call last):
  File "/home/docs/checkouts/readthedocs.org/user_builds/pfsspy/checkouts/362/examples/using_pfsspy/plot_aia_overplotting.py", line 13, in <module>
    import sunpy.io.fits
ModuleNotFoundError: No module named 'sunpy.io.fits'