Some type conversions result in different results in Dolt than MySQL

We had a large set of type alteration tests that relied on custom logic. We changed how we handle type changes to put it under the control of the SQL engine, which changed how some of these conversions behaved. As a result these tests have been disabled for close to 2 years. This PR deletes them:

https://github.com/dolthub/dolt/pull/6733

The right place for these tests is in GMS as engine tests, where they can run on both the memory implementation and dolt. This issue is a reminder to port these tests there and fix the GMS type conversion logic to match MySQL's in all cases.