boards: arm: nrf: Remove Kconfig PWM_[0-3] usage

The Kconfig PWM_[0-3] sybmols don't have any meaning for nrf family of
SoCs.  The driver doesn't utilize them and no sample or test code does
either so we can remove setting them in board Kconfig.defconfig files.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>