btrfs: don't print 'has skinny extents' anymore on mount

Skinny extents have been a default mkfs feature since version 3.18 i
(introduced in btrfs-progs commit 6715de04d9a7 ("btrfs-progs: mkfs:
make skinny-metadata default") ). It really doesn't bring any value to
users to simply remove it.

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