PCI: endpoint: Make struct pci_epf_driver::remove return void

The driver core ignores the return value of pci_epf_device_remove()
(because there is only little it can do when a device disappears) and
there are no pci_epf_drivers with a remove callback.

So make it impossible for future drivers to return an unused error code
by changing the remove prototype to return void.

The real motivation for this change is the quest to make struct
bus_type::remove return void, too.

Acked-by: Kishon Vijay Abraham I <kishon@ti.com>
Acked-by: Bjorn Helgaas <bhelgaas@google.com>
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Link: https://lore.kernel.org/r/20210713193522.1770306-2-u.kleine-koenig@pengutronix.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>