btrfs: remove unused pointer inode in relink_file_extents

Pointer inode is being assigned but is never used hence it is redundant
and can be removed. It's been unused since the introduction in
38c227d8 ("Btrfs: snapshot-aware defrag").

Cleans up clang warning:

  variable ‘inode’ set but not used [-Wunused-but-set-variable]

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>