flash: nxp: removal of deprecated binding nxp,lpc-iap

The binding 'nxp,lpc-iap' is no longer used, which is confirmed
by running:

$ find ${ZEPHYR_BASE}/dts/arm/nxp -type f | egrep -e '\.dts(i)*$' | \
  xargs grep -nH nxp,lpc-iap

Changes in this commit:
 - remove DT_HAS_NXP_LPC... in drivers/flash/Kconfig.mcux
 - remove schema file for nxp,lpc-iap

Signed-off-by: Yves Vandervennet <yves.vandervennet@nxp.com>