usb: phy: phy-msm-usb: Remove redundant extcon register/unregister

Since usb phy core has added common code to register or unregister
extcon device, then phy-msm-usb driver does not need its own
code to register/unregister extcon device, then remove them.

Signed-off-by: Baolin Wang <baolin.wang@linaro.org>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>