Remove duplicate of add_gas_switch_event in dive.c

Signed-off-by: Anton Lundin <glance@acc.umu.se>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Use the get_o2/get_he helpers

We have helpers that take care of O2_IN_AIR and so on.

Signed-off-by: Anton Lundin <glance@acc.umu.se>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>