boards/stm32f3discovery: Fix no external low speed crystal

The board does not have external low speed crystal.

Page 20 of User Manual: 6.10.2 "OSC 32 kHz clock supply"

 * X3 Crystal (not provided)

https://www.st.com/resource/en/user_manual/dm00063382.pdf

This fixes `example/default` and makes `tests/periph_rtc` work properly.