Remove JpegImageFile.load_djpeg()

[akx]

Sibling of #9901 (_save_netpbm() for GIFs) and #9084 (save_cjpeg() for JPEGs).

This PR proposes to remove the undocumented JpegImageFile.load_djpeg() function and the testing infrastructure for it.

[radarhere]

Could you make an argument for why this should be removed, rather than documented?

[akx]

@radarhere Sure! _save_cjpeg(), the save counterpart of this function, was removed without much ceremony by you in #9084.

This function might have had some value if it was automatically put into place if actual JPEG loading functions weren't available, but that has never been the case, so, like #9084, I'd call this unused.

To match Hugo's research comment for #9084:

• This was added (unused, untested and undocumented) in PIL 1.1.1 (2000): hugovk/PIL@44fbc52
• It has since then materially been touched thereafter only to
  • Fix security issues related to its insecure tempfile handling and shell injection vulnerabilities (Fix for insecure use of tempfile.mktemp #548 and Djpeg #748)
  • To fix it so the test added in the aforementioned security commit works on Windows in commit c0fb5ac.
• The only callers for this are tests: the aforementioned security test and one added in d283f77.

Also, there's this 2014 comment expressing desire to get rid of this function, so maybe a decade and change later that could be done. #731 (comment)

EDIT: GitHub Code Search seems to find a single call from 10 years ago, from a seemingly abandoned mirror of https://launchpad.net/qa-regression-testing.

[radarhere]

I think _save_cjpeg() starting with an underscore is a difference. I would suggest a deprecation, rather than a removal.

[akx]

Alright, I'll rework it to a deprecation when I get to a real terminal. But with that rationale, #9901 should be OK to merge?