remove deprecated lints in 3.0

[pq]

3.0 would be a good time to remove deprecated lints.

Candidates:

• remove avoid_as #59008
• remove invariant_booleans #59011
• remove prefer_bool_in_asserts #59012
• remove prefer_equal_for_default_values #59013
• remove super_goes_last #59014
• remove enable_null_safety #59017
  • (replaced by Analyzer doesn't error on legacy libraries #50694)

[srawlins]

I think enable_null_safety should also be removed in 3.0. This rule is being replaced by #50694.

[srawlins]

I'd also like to remove avoid_null_checks_in_equality_operators, which mostly doesn't make sense under null safety, with a proposed rule: equality_operator_override_with_nullable_parameter

[srawlins]

None of these have to do with null safety, so they can be deleted even before Google internal is null safe.

(The last one, enable_null_safety, does obviously pertain to null safety, but is not used internally, so it can also be deleted.)