ath79: mikrotik: fix build error with kernel 6.1

ath79: mikrotik: fix build error with kernel 6.1

drivers/mfd/rb4xx-cpld.c:167:19: error: initialization of 'void (*)(struct spi_device *)' from incompatible pointer type 'int (*)(struct spi_device *)' [-Werror=incompatible-pointer-types]
  167 |         .remove = rb4xx_cpld_remove,
      |                   ^~~~~~~~~~~~~~~~~
drivers/mfd/rb4xx-cpld.c:167:19: note: (near initialization for 'rb4xx_cpld_driver.remove')
cc1: some warnings being treated as errors
make[8]: *** [scripts/Makefile.build:250: drivers/mfd/rb4xx-cpld.o] Error 1
make[7]: *** [scripts/Makefile.build:500: drivers/mfd] Error 2
make[6]: *** [scripts/Makefile.build:500: drivers] Error 2
make[5]: *** [Makefile:2012: .] Error 2

As the allocated function does nothing, simply delete it.

Signed-off-by: Koen Vandeputte <koen.vandeputte@citymesh.com>