btrfs: Remove redundant forward declarations

Some static functions are needlessly forward declared. Let's remove those
declarations since they add no value.

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