ARM: dts: imx50: remove non-existent USB instances

According to the i.MX50 Reference Manual, the SoC features one USB OTG
and one host controller. Remove the non-existent instances from the
DTS which probably got initially copied from the mx53 DTS.

Signed-off-by: Alexander Kurz <akurz@blala.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>