btrfs: Remove unused parameters from various functions

iterate_dir_item:found_key - introduced in 31db9f7c ("Btrfs:
  introduce BTRFS_IOC_SEND for btrfs send/receive"), yet never used.

record_ref:num - ditto

This is a first pass with the low-hanging fruit. There are still quite a
few unsued parameters in some function which have to abide by a callback
interface.

Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: Josef Bacik <jbacik@fb.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>