boards: arm: Remove redundant CONFIG_BOARD_* from imx rt defconfigs

The Kconfig board choice only has only one option which is implicitly
enabled, therefore it is not necessary to set the symbol explicitly in
board defconfigs.

mimxrt10{50,60}_evk boards are excluded from this change because they
have multiple board configurations (e.g., hyperflash vs. qspi) in the
board choice.

Signed-off-by: Maureen Helm <maureen.helm@nxp.com>