[conf] remove non-functional FIRMWARE define

Defines don't work this way...
Was only used in gps_ubx_ucenter, replace it with GPS_UBX_UCENTER_SLOW_NAV_SOL flag
to set a lower NAV_SOL message rate (currently not set anywhere).
So you get full NAV_SOL messages rate for fixedwings as well
(just like before since the test for rotorcraft firmware was non-functional and always true).