vehicle_gps_position: remove timestamp_variance & timestamp_velocity (they're not used)

msg/vehicle_gps_position.msg

src/drivers/gps/devices

src/drivers/gps/gps.cpp

src/modules/ekf2/ekf2_main.cpp

src/modules/mavlink/mavlink_receiver.cpp

.../position_estimator_inav/position_estimator_inav_main.cpp

src/modules/uavcan/sensors/gnss.cpp

src/platforms/posix/drivers/gpssim/gpssim.cpp