Stop JDBC drivers shipping with Roo (they're available via RooBot)

Roo 1.1.0 shipped selected JDBC drivers for DBRE purposes such as Postgres and MySql. Community feedback indicated it was too complicated to install other JDBC drivers into Roo (although we did publish a script to simplify this). As such ROO-1820 provided a far more elaborate and extensible mechanism for discovering JDBC drivers available for automatic installation in Roo. This even allows third parties to package new JDBC drivers and have them detected by Roo (they just need to register those JDBC drivers as an add-on with RooBot). Accordingly we can end the shipment of JDBC drivers with Roo itself, given all are now so easy to install.