btrfs: remove no longer used btrfs_migrate_to_delayed_refs_rsv()

The function btrfs_migrate_to_delayed_refs_rsv() is no longer used.
Its last use was removed in commit 2f6397e4

 ("btrfs: don't refill
whole delayed refs block reserve when starting transaction").
So remove the function.

Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>