rtc: sh: remove unused variable rtc_dev

When building rtc-sh, rtc_dev isn't used in function __sh_rtc_periodic.
drivers/rtc/rtc-sh.c: In function ‘__sh_rtc_periodic’:
drivers/rtc/rtc-sh.c:146:21: warning: unused variable ‘rtc_dev’ [-Wunused-variable]
  struct rtc_device *rtc_dev = rtc->rtc_dev;
                     ^~~~~~~
Remove the declaration of rtc_dev to dispose the warning.

Fixes: ec623ff0 ("rtc: sh: remove dead code")
Signed-off-by: Anders Roxell <anders.roxell@linaro.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>