[ahrs] pass dt to ahrs_propagate

not really properly taken into account for:
- int_cmpl_euler: this is very ugly and still has F_UPDATE 512Hz hardcoded, but nobody should use this one anyway..
- float_cmpl and int_cmpl_quat: still use AHRS_PROPAGATE_FREQUENCY to set the gains...
[conf] remove some unneeded defines
[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)