pm: remove PM_DEVICE_ACTION_FORCE_SUSPEND

PM_DEVICE_ACTION_FORCE_SUSPEND has been equivalent to
PM_DEVICE_ACTION_SUSPEND for a while. Release notes for Zephyr 2.7
mentioned its removal (see 76702e8f) but
for some reason it was kept.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>