configure.ac: remove no longer needed *_pci_id logic

configure.ac: remove no longer needed *_pci_id logic

Previously it was used to differentiate between the different codepaths
in the loader. Although strictly speaking the (core) of the loader is
only used when a hardware device is available. The latter of which in
itself requires libdrm (one of the codepaths available).

That said, all the configure toggles which relate to enabling/using hw
device should attribute and require libdrm, so there's no need to keep
this code around.

With this gallium_require_drm_loader becomes an empty stub, so nuke that
one as well.

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Axel Davy <axel.davy@ens.fr>
Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>