drivers: adc: stm32: move internal path setting to sensor drivers

On some STM32 families (such as F4), temperature and Vbat sensor share the
same ADC channel, which can lead to conflict when reading them, and wrong
measurement can follow.

To alleviate this problem, this commit moves the setting of the common
path internal channel to the sensor drivers themselves instead of doing
it in the ADC driver.

The teardown is still done in the ADC driver, systematically, instead of
channel by channel (which has the same result).

By moving this logic in the sensor drivers, the properties temp-channel,
vbat-channel and vref-channel becomes useless and are thus removed.

Signed-off-by: Guillaume Gautier <guillaume.gautier-ext@st.com>