btrfs: don't print 'flagging with big metadata' anymore on mount

Added in commit 727011e0 ("Btrfs: allow metadata blocks larger than
the page size") in 2010 and it's been default for mkfs since 3.12
(2013).  The message doesn't really convey any useful information to
users. 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>