dwc3: Remove unnecessary phy shutdown callbacks

dwc3 core driver might be probed/removed due to probe deferrals. As
core driver does not call phy init during probe there is no need to
call shutdown during remove which can result in regulators imbalance
and charger detection failure. Phy init and shutdown are handled from
glue driver.

Change-Id: I63747961629f90498a75f636f54cd8a54062f29f
Signed-off-by: Vamsi Krishna Samavedam <vskrishn@codeaurora.org>