vfio: Delete the unbound_list

commit 60720a0f ("vfio: Add device tracking during unbind") added the
unbound list to plug a problem with KVM where KVM_DEV_VFIO_GROUP_DEL
relied on vfio_group_get_external_user() succeeding to return the
vfio_group from a group file descriptor. The unbound list allowed
vfio_group_get_external_user() to continue to succeed in edge cases.

However commit 5d6dee80

 ("vfio: New external user group/file match")
deleted the call to vfio_group_get_external_user() during
KVM_DEV_VFIO_GROUP_DEL. Instead vfio_external_group_match_file() is used
to directly match the file descriptor to the group pointer.

This in turn avoids the call down to vfio_dev_viable() during
KVM_DEV_VFIO_GROUP_DEL and also avoids the trouble the first commit was
trying to fix.

There are no other users of vfio_dev_viable() that care about the time
after vfio_unregister_group_dev() returns, so simply delete the
unbound_list entirely.

Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
Acked-by: Alex Williamson <alex.williamson@redhat.com>
Link: https://lore.kernel.org/r/20220418005000.897664-10-baolu.lu@linux.intel.com
Signed-off-by: Joerg Roedel <jroedel@suse.de>