soc: arm: st_stm32: l0: remove unsupported state

SOFT_OFF state (which translates to standby) doesn't seem to be
supported according to the L0 DT files. Also, by definition soft off
implies context loss, ie boot from scratch, but the implementation
expected a call to the PM exit hook.

Signed-off-by: Gerard Marull-Paretas <gerard@teslabs.com>