Remove usage of most code specific to Django 1.6.
Review Request #11347 — Created Dec. 23, 2020 and submitted
While porting over to Django 1.11, we had a lot of code blocked off by
version checks. As this is no longer needed, this code has been removed,
slimming things down just a bit.
There are still places where we call compatibility functions, or do
things in a legacy way, and there's likely other walled-off code that
uses something other than version checks, but this gets us a good step
of the way toward eliminating all that.
Unit tests pass.