ARM: dts: imx6qdl-nitrogen6x: remove duplicate iomux entry

The NANDF_CS2 pad is also part of the wlan-vmmcgrp iomux group.

Removing is from the usdhc2grp group avoids the following error:
imx6q-pinctrl 20e0000.iomuxc: pin MX6Q_PAD_NANDF_CS2 already requested
by regulators:regulator@4; cannot claim for 2194000.usdhc
imx6q-pinctrl 20e0000.iomuxc: pin-187 (2194000.usdhc) status -22
imx6q-pinctrl 20e0000.iomuxc: could not request pin 187
(MX6Q_PAD_NANDF_CS2) from group usdhc2grp on device 20e0000.iomuxc

Signed-off-by: Gary Bisson <gary.bisson@boundarydevices.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>