Btrfs-progs: remove some dead/unbuilt code

Btrfs-progs: remove some dead/unbuilt code

Remove some commented-out & #if 0'd code:

 * close_blocks()
 * btrfs_drop_snapshot()
 * btrfs_realloc_node()
 * btrfs_find_dead_roots()

There are still some #if 0'd functions in there, but I'm hedging
on those for now, they have been copied to cmds-check.c and I want
to see if they can be brough back into ctree.c eventually.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>