btrfs: Clean up unused variables in free-space-tree.c

Remove variables 'start' and 'end', which are set but never used.

Signed-off-by: Christos Gkekas <chris.gekas@gmail.com>
Reviewed-by: Omar Sandoval <osandov@fb.com>
Signed-off-by: David Sterba <dsterba@suse.com>