Remove remaining mysql cruft

MySQL support was removed many months ago as we chose PostgreSQL. 

It turns out, these two methods become a lot more readable without all of the switching on adapter type.

:smiley:  :cake:

I chose not to remove the the remaining references to MySQL in [active_record_query_parts.rb](https://github.com/ManageIQ/manageiq/blob/master/vmdb/lib/active_record_query_parts.rb) because that might be useful to be database agnostic elsewhere.  Additionally, I didn't touch the old migrations.