ARM: dts: imx6sl-warp: Remove USB regulators

According to the warp schematics there are no regulators for the USB OTG
ports, so let's remove them.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Otavio Salvador <otavio@ossystems.com.br>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>