rtc: at91rm9200: annotate at91_rtc_remove with __exit again

Having the driver as a builtin after dropping the __exit annotation results
in:

>> drivers/rtc/rtc-at91rm9200.c:561:13: warning: 'at91_rtc_remove' defined but not used [-Wunused-function]
     561 | static void at91_rtc_remove(struct platform_device *pdev)
         |             ^~~~~~~~~~~~~~~

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202310160705.vlElNOGb-lkp@intel.com/
Link: https://lore.kernel.org/r/20231016041816.246426-1-alexandre.belloni@bootlin.com


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