usb: hub: remove CONFIG_USB_HUB_MIN_POWER_ON_DELAY

Now that we wait the correct specification-mandated time at the end of
usb_hub_power_on(), I suspect that CONFIG_USB_HUB_MIN_POWER_ON_DELAY has
no purpose.

For cm_t35.h, we already wait longer than the original MIN_POWER_ON_DELAY,
so this change is safe.

For gw_ventana.h, we will wait as long as the original MIN_POWER_ON_DELAY
iff pgood_delay was at least 200ms. I'm not sure if this is the case or
not, hence I've CC'd relevant people to test this change.

Cc: Igor Grinberg <grinberg@compulab.co.il>
Cc: Tim Harvey <tharvey@gateworks.com>
Signed-off-by: Stephen Warren <swarren@nvidia.com>