btrfs: make btrfs_free_device() static

The function btrfs_free_device() is never used outside of volumes.c, so
make it static and remove its prototype declaration at volumes.h.

Reviewed-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>