Remove duplicate schema management tests

[morozov]

Removed more schema management integration tests:

1. NewPrimaryKeyWithNewAutoIncrementColumnTest duplicates AlterTableTest#testAddPrimaryKeyOnNewAutoIncrementColumn() but runs only on MySQL
2. DBAL168Test doesn't fail if the code change it covers is reverted, although other tests do
3. DBAL510Test doesn't cover any specific requirement

The code coverage is reported as slightly dropped but it looks like a false positive:

Commits have different number of coverage report uploads

The no longer covered lines are unrelated to the tests being removed and must still be covered by other tests.