smbus: remove smbus_trans / SMB_TRANS

This change reverts most of r281985.

The method did not map to anything defined by SMBus protocol and could
not be implemented for SMBus controllers.

This change is obviously not backwards compatible, but I have good
reasons to believe that there have never been any users of SMB_TRANS.

Discussed with: grembo, jhb
MFC after: 6 weeks