Nokia RX-51: Remove CONFIG_PREBOOT from defconfig

CONFIG_PREBOOT just cause putting "preboot=CONFIG_PREBOOT" into env list.
Value CONFIG_PREBOOT="run preboot" in defconfig is just nonsense and does
not do anything useful (it is infinite recursion). Config file for this
board already contains default preboot= env variable with correct value,
which has higher priority than CONFIG_PREBOOT and this is reason why
nonsense CONFIG_PREBOOT is ignored.

Remove nonsense and unused CONFIG_PREBOOT from nokia_rx51_defconfig file.

Signed-off-by: Pali Rohár <pali@kernel.org>