char: rtc: remove unused rtc_control() API

Since commit 34ce71a9 ("ALSA: timer: remove legacy rtctimer"), the
rtc_register/rtc_control/rtc_unregister API is unused. As it is highly
unlikely to be needed again, remove it.

Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>