Unify `pastis_contrast()` in `util` for all telescopes

[ivalaginja]

The function pastis_contrast() in util_pastis.py currently needs to return result.value when running for JWST and result when running for LUVOIR, so that it gets stripped of its astropy unit. This needs to be made independent of the telescope that is being worked on.

[ivalaginja]

This doesn't appear to be a bug anymore, although I'd like to spend a moment on confirming this.