[ins] alt_float: remove ALT_KALMAN_ENABLED

There is really no benefit in keeping the ALT_KALMAN_ENABLED define (and setting it to FALSE by default).
If USE_BAROMETER alt_kalman is always enabled and if you don't want to use altitude/climb_rate filtering,
use the ins type "gps_passthrough".
Now if you use the alt_float INS, it's always enabled (which is what most people expect anyway).

In theory it was possible to disable the alt_kalman in flight before (if ! USE_BAROMETER),
but I very much doubt that was used since there also was no settings file for it.