rtc: remove rtc-at32ap700x

Since PLATFORM_AT32AP is an AVR32 platform which was removed, the
rtc driver rtc-at32ap700x is useless.
This patch remove it.

Signed-off-by: Corentin Labbe <clabbe.montjoie@gmail.com>
Acked-by: Hans-Christian Noren Egtvedt <egtvedt@samfundet.no>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>