Remove deprecated MigrateUpgradeTestBase

This is a test, unused by contrib. No need to keep it around http://grep.xnddx.ru/search?text=MigrateUpgradeTestBase

Normally we treat test base classes as API. However:

• As stated, this test is not used in contrib.
• The only things that would break if we remove this are tests of custom migration UIs built on top of the core UI (which likely don't exist) or tests of custom migrations that for some reason were written as UI tests.
• In both cases, the tests also would have been written before Migrate UI was even stable.
• And, of course, it doesn't affect runtime.

So in this case I'm comfortable removing this. Also nice to get rid of it since it's a WTB test and all.

I do think we should add a small change record for the removal though, on the off chance that someone out there does depend on this somehow, since it is technically a BC break. It should be committed to 8.8.x only.

I think @catch agrees on removing it since he pinged me asking but he didn't explicitly say he was +1 also, so I or he can get confirmation of that before commit.

Back to rtbc. Despite the +1 from @xjm, I'm tagging for RM review just to make sure @catch has a chance to voice any objections per #4 and @catch and @xjm can coordinate the commit.

Yes just to confirm I am indeed +1 it's just good with borderline things to get a second opinion so we're as consistent as possible.

Committed 1fd92fc and pushed to 8.8.x. Thanks!