drivers: i2c: stm32: drop few redundant guards in RTIO driver

Drop few CONFIG_PM_DEVICE_RUNTIME guards, the pm_device_runtime
functions they are masking are no-op automatically when the
corresponding config option is not selected.

This change is related to the change made in the STM32 non-RTIO driver
trough commit 4f3523d9 ("i2c: stm32: drop few redundant guards").

Signed-off-by: Etienne Carriere <etienne.carriere@st.com>