rtc: stm32: remove one condition check in stm32_rtc_set_alarm()

A condition check was repeated in this function implementation despite of
a corresponding check in the stm32_rtc_alarm_irq_enable() function.
Thus delete redundant source code here.

This issue was detected by using the Coccinelle software.

Suggested-by: Russell King <linux@armlinux.org.uk>
Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Reviewed-by: Amelie Delaunay <amelie.delaunay@st.com>
Link: https://lore.kernel.org/r/4da614a4-83c6-548c-a112-033b846c561b@web.de
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>