board: st: Remove board_early_init_f and board_late_init callbacks for stm32 boards

Remove board_early_init_f() and board_late_init() callbacks for stm32
boards as the corresponding flags (CONFIG_BOARD_LATE_INIT and
CONFIG_BOARD_EARLY_INIT_R) are now disabled.

Signed-off-by: Patrice Chotard <patrice.chotard@foss.st.com>
Reviewed-by: Patrick Delaunay <patrick.delaunay@foss.st.com>