rtc: Remove unused rtc_dev_exit().

Commit 270a3bd6 ("rtc: make class.c explicitly non-modular")
removed rtc_dev_exit() call.

Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/1e2f9a99-435d-e149-7456-dfacd53be85a@I-love.SAKURA.ne.jp