drivers: timer: stm32: Clean up related to STM32_LPTIM_CLOCK

Now that STM32_LPTIM_CLOCK choice symbol is defined from device tree,
remove the prompt which was defining it as a user selectable entry.

Remove the warning related to possible symbol misalignment with
device tree setting.

Signed-off-by: Erwan Gouriou <erwan.gouriou@st.com>