char: rtc: remove task handling

Since commit 9e7002a7 ("char: rtc: remove unused rtc_control() API"),
it is not possible to set a callback anymore, remove its handling from the
interrupt handler.

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