boot: zephyr: boards: delete mcxn947 configuration.

- Deletes configuration for the FRDM-MCXN947 and MCX-N9XX-EVK boards.
- Enables MCU-Boot default Swap mode for the FRDM-MCXN947 and MCX-N9XX-EVK,
  after the flash driver was upgraded to support the 16-byte programming.

Signed-off-by: Andrej Butok <andrey.butok@nxp.com>