drivers: serial: Remove SOC_NRF52810 Kconfig reference
drivers: serial: Remove SOC_NRF52810 Kconfig reference

This symbol will be added by
https://github.com/zephyrproject-rtos/zephyr/pull/7915.

Having it in there doesn't hurt that much by itself (undefined symbols
default to 'n'), but I'm about to turn references to undefined Kconfig
symbols turn into an error.

Remove the reference.

Signed-off-by: Ulf Magnusson <01743f1c1deb1fe75271d62edb65757d98ac0bcb@nordicsemi.no>

kraj/zephyr,Vudentz/zephyr,explora26/zephyr,GiulianoFranchetto/zephyr,finikorg/zephyr,kraj/zephyr,ldts/zephyr,nashif/zephyr,nashif/zephyr,nashif/zephyr,punitvara/zephyr,punitvara/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,ldts/zephyr,galak/zephyr,punitvara/zephyr,finikorg/zephyr,nashif/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,ldts/zephyr,finikorg/zephyr,GiulianoFranchetto/zephyr,punitvara/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,punitvara/zephyr,explora26/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,ldts/zephyr,explora26/zephyr,kraj/zephyr,GiulianoFranchetto/zephyr,ldts/zephyr,GiulianoFranchetto/zephyr,finikorg/zephyr,kraj/zephyr,kraj/zephyr,nashif/zephyr,galak/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,explora26/zephyr,explora26/zephyr,galak/zephyr,finikorg/zephyr,galak/zephyr,galak/zephyr,Vudentz/zephyr