usb: renesas-xhci: Remove renesas_xhci_pci_exit()

Remove empty function renesas_xhci_pci_exit() that does not
actually do anything.

Cc: Mathias Nyman <mathias.nyman@intel.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Vinod Koul <vkoul@kernel.org>
Signed-off-by: Moritz Fischer <mdf@kernel.org>
Link: https://lore.kernel.org/r/20210718015111.389719-3-mdf@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>