imx: mx6ul_14x14_evk: prepare to synchronise device trees with linux

As a preparatory step remove the qspi node with its flash0 label as this
is already in the Linux upstream device tree.

This avoids the following error once synchronised:

+arch/arm/dts/imx6ul-14x14-evk.dtb: ERROR (duplicate_label):
 /soc/bus@2100000/spi@21e0000/n25q256a@0: Duplicate label 'flash0' on
 /soc/bus@2100000/spi@21e0000/n25q256a@0 and
 /soc/bus@2100000/spi@21e0000/flash@0

Signed-off-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>