Accelerometer-based velocity values are invalid after apogee

Accelerometer-based velocity values are invalid after apogee

Because the orientation of the flight computer relative to the ground is
unknown after apogee, the accelerometer data cannot be integrated to compute
velocity. Main deploy is now based purely on barometric altitude and landing
detection no longer checks for a low velocity value.

Signed-off-by: Keith Packard <keithp@keithp.com>