arm64: dts: imx8mn: Remove setting for IMX8MN_CLK_USB_CORE_REF

Since IMX8MN_CLK_USB_CORE_REF is not used at all, so remove the setting
for it.

Signed-off-by: Li Jun <jun.li@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>