rtc: MAINTAINERS: drop Alessandro Zummo

Last email from Alessandro was in 2016, so remove him from maintainers
of the RTC subsystem.  Stale maintainer entries hide information whether
subsystem needs help, has a bus-factor or is even orphaned.

Link: https://lore.kernel.org/all/?q=f%3A%22Alessandro+Zummo%22
Cc: Alexandre Belloni <alexandre.belloni@bootlin.com>
Cc: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Alessandro Zummo <a.zummo@towertech.it>
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Link: https://lore.kernel.org/r/20231211132600.101090-1-krzysztof.kozlowski@linaro.org
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>

Author: krzk

MAINTAINERS

@@ -18286,7 +18286,6 @@ X:	include/linux/srcu*.h
 X:	kernel/rcu/srcu*.c
 
 REAL TIME CLOCK (RTC) SUBSYSTEM
-M:	Alessandro Zummo <a.zummo@towertech.it>
 M:	Alexandre Belloni <alexandre.belloni@bootlin.com>
 L:	linux-rtc@vger.kernel.org
 S:	Maintained