gem(4): Remove onboard-only Sun ERI and remnants of SBus support

These bits are obsolete since 58aa35d42975c298ca0adba705c042596303c9f5.
This change reverts part of 9ba2b298dfda78b6669281507b314a4c441cd706 as
well as effectively bd3d9826d73b780d349c53ee373060bc7b8d0581, i. e. the
SBus-related modifications. This also gets rid of a nasty hack required
as bus_{read,write}_N(9) doesn't really fit bus_space_subregion(9).