usb: renesas_usbhs: Remove dummy runtime PM callbacks

Platform drivers don't need dummy runtime PM callbacks that just return
success in order to have runtime PM happening. This has changed since
following commits:

commit 05aa55dd ("PM / Runtime: Lenient generic runtime pm callbacks")
commit 543f2503 ("PM / platform_bus: Allow runtime PM by default")
commit 8b313a38 ("PM / Platform: Use generic runtime PM callbacks directly")

Signed-off-by: Jarkko Nikula <jarkko.nikula@linux.intel.com>
Reviewed-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Acked-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
[shimoda: revise git commit description style]
Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>