boards: arm: Remove nonexistent Kconfig symbols

CONFIG_BOARD_STM32_NUCLEO_{L432KC,L476RG} where set in the respective
board defconfig files, however these symbols don't exist.  We can just
remove them from the defconfig as we'll get the board Kconfig symbol
defined by default when selecting that board.

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