[rotorcraft] ahrs dt: only calc if USE_AUTO_AHRS_FREQ or !defined(AHRS_PROPAGATE_FREQUENCY)
[fixedwing] remove reduced propagation rate stuff

Also calculate dt if USE_AUTO_AHRS_FREQ or !defined(AHRS_PROPAGATE_FREQUENCY)
[test] update some mostly unused ahrs tests
[ahrs] also pass dt to update_accel and update_mag

Not so sure passing dt to the update functions is the best way to go...
Is only used to update the gains of the cmpl filters right now.
But it makes it possible to properly compute the gains and avoids having a dependency on sys_time in the AHRS.
[rotorcraft] only calc mag dt if actually using mag
[fixedwing] minor cleanup for ahrs_sim
[ins] alt_float, bound dt and used fixed GPS_DT if defined
[airborne] calc dt with usec resolution