Do not use CurveLoop in RevitGeometryFromMirrorData method

### Issue

Geometry display in the Revit viewport, the so called "GeomKeeper" API, was crashing repeatedly in a handful of workflows.  These workflows had the following in common:

1)  Usage of `PolyCurve`
2)  Usage of `Solid`

Alley discovered that the issue was likely having to do with memory corruption of Revit `Curve` objects passed to the GeomKeeper Element.  This is surprising, given that we were already using the standard Revit `Curve` creation APIs quite extensively without issue.

### Bug Origin

You can consider this a stop-gap fix.  We're still not totally sure what the underlying issue is, but it likely has to do with `CurveLoop`.  

In `RevitGeometryFromMirrorData`, `PolyCurve`'s are converted into `CurveLoop`'s by first the `ToRevitType` extension method.  Beyond the fact that there is no reason to create a `CurveLoop` here (which guarantees the collection of `Curve`'s is closed, not guaranteed by `PolyCurve`), we have reason to believe that enumerating the `Curve`'s in the `CurveLoop` (`CurveLoop` implements `IEnumerable<Curve>`) transfers ownership of the underlying native `Curve` incorrectly.  In short, the managed GC cleans up the `Curve`, corrupting the native `Curve`.  This is just a hunch, though.

### Fix 

By removing the usage of `CurveLoop`, the crash is now unreproducible.

### Reviewers 

@ikeough
This was introduced just as a readability fix.  I don't think it is related.