drivers: flash: soc_flash_mcux: remove CMD_MARGIN_CHECK

The CMD_BLANK_CHECK can return errors when the flash is readable, and
should only be used after programming, not in is_area_readable().  From
the LPC55S69 datasheet: "As cells age and lose charge, a correctly
programmed address will fail this check, while still being able to be
read successfully for the remaining duration of the data retention time."

Signed-off-by: Derek Snell <derek.snell@nxp.com>