btrfs: remove unused bio_flags argument to btrfs_submit_metadata_bio

btrfs: remove unused bio_flags argument to btrfs_submit_metadata_bio

This argument is unused since commit 44ec8d9c7fbf ("btrfs: factor out
helper adding a page to bio") and commit 78072098914d ("btrfs: call
submit_bio_hook directly for metadata pages") reworked the way metadata
bio submission is handled.

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