PR #619 completely removed support for `compare_against_backend`, but it's still documented

[CaselIT]

The PR #619 has completely removed support for compare_against_backend, but it's still documented and supposedly tested (the test is clearly broken.)

@zzzeek This seems to be a rarely used feature, since no-one reported it, but since it's still documented and all it may be better to just re-introduce it. If you feel like it make more sense removing it, then this issue could be used to remove the docs for it

Originally posted by @CaselIT in #619 (comment)

[zzzeek]

we never fixed this; removing from the docs now

[sqla-tester]

Mike Bayer has proposed a fix for this issue in the main branch:

remove compare_against_backend from tests and docs https://gerrit.sqlalchemy.org/c/sqlalchemy/alembic/+/4925