boards: stm32h7: Remove lptim related configuration

No power.c implementation is provided for stm32h7 soc series.
Since there is no code provided to handle entry and exit of stop states or
PM use case globally, there is no reason to provide a configuration of
LPTIM which only current use is to be used as PM ticker.

Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>