pci.h: Drop unused `mainboard_pci_subsystem*` prototypes

We used to allow mainboards to override subsystems using
mainboard_pci_subsystem_vendor_id and mainboard_pci_subsystem_device_id.

Mechanisms have changed and the only occurrence of these names is in
the header.

Change-Id: Ic2ab13201a2740c98868fdf580140b7758b62263
Signed-off-by: Patrick Georgi <patrick@georgi-clan.de>
Reviewed-on: http://review.coreboot.org/2625
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Tested-by: build bot (Jenkins)
1 file changed