rtc: sh: remove periodic interrupt handling

Because periodic interrupts are emulated by the RTC core, the PIE
handling code can simply go away now. And with it the custom proc-file.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Link: https://lore.kernel.org/r/20250227134256.9167-14-wsa+renesas@sang-engineering.com


Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>