Remove AbstractMysqlAdapterTest

This test was added in 8f5095a to ensure that there was coverage
for `AbstractMysqlAdapter#execute`, but 63c0d6b refactored `#execute`
to be defined at the Abstract adapter level, and rely on concrete MySQL
adapters to implement `#raw_execute`.

This test won't pass without having the `ExampleMysqlAdapter` implement
`#raw_execute`, but this test is obsolete now that AbstractMysql2Adapter
doesn't implement `#execute` directly.

Author: adrianna-chang-shopify