Remove imported database drivers

[titpetric]

While supporting pgsql/sqlite/mysql, we don't need (or want) to bundle the drivers for them. E.g. there are multiple choices for drivers for any given database, and a mysql driver may not be the one which is currently bundled. Since the driver registration api is done with init(), that becomes a bit of a mess.

Fix: remove bundled database drivers

[titpetric]

We still need to import mysql, as we use MYSQLError (wouldn't like to resort to parsing it)