btrfs: remove no longer used btrfs_transaction_in_commit()

The function btrfs_transaction_in_commit() is no longer used, its last
use was removed in commit 11aeb97b

 ("btrfs: don't arbitrarily slow
down delalloc if we're committing"), so just remove it.

Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>