Support removing unique constraints

As of now we have the following error message:

```
unimplemented: cannot drop UNIQUE constraint "uniq_rails_1e07660b77" using ALTER TABLE DROP CONSTRAINT, use DROP INDEX CASCADE instead
```

It seems that with a local version of the `#remove_constraint` method we could support it.

See excluded test `test_remove_unique_constraint_by_column` (as of writting, this exclusion is part of the rails 7.1 PR).