boards: arm: b_l072z_lrwan1: Use LSE for RTC source clock

* This board was originally configured through its Kconfig to use LSE
instead of LSI for the RTC source clock, so the dts is updated
accordingly.
* Remove the RTC source clock symbol from the Kconfig since it is now
deprecated.

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