Didn't touch comments where it wasn't obvious that the code adhered to
the newer standard.

Thanks Maxime Lorant for the report and Anssi for the suggestion.

It's no longer necessary for GenericForeignKey (and any other virtual fields)
to intercept the field's values using the pre_init signal.

Thanks Burhan Khalid for the initial patch and knbk/timgraham
for review.

Refs #18149.

Refs #26341.

Thanks Sylvain Garancher for the report and Veranika Sabiashchanskaya for the
initial patch.

Refs #17375.

Thanks iktyrrell for the patch.

RendererMixin will soon be removed but this removal and the corresponding
test changes stand on their own.

Thanks Collin for the report.

Thanks Tim for the suggestion.