Remove REFLINE from masks

I referred to this problem in https://pfspipe.ipmu.jp/jira/browse/PIPE2D-1389, but it didn't get picked up.

For internal reasons, we set a REFLINE bit wherever there is or might be a reference line,
which (especially in the NIR) can be most pixels. It means that any user wanting to use the spectral masks to reject bad pixels has to explicitly mask out this bit; (spec.mask & ~spec.masks.flags["REFLINE"]) != 0x0 or something like that.

I think we should clear this bit, or at the very least provide a way for users to easily mask out all bad pixels, as judged by the PFS team. I'd go with the former myself; I have never had cause to use REFLINE while looking at PFS data.