Remove broken asserts in PIFLoadingTests

[owenv]

These asserts never worked and aren't useful, but we're inactive until recently due to an erroneous nil check. Drop them from the test entirely

[owenv]

@swift-ci test

[mhrawdon]

I think the intent of these assertions was to check that the build phases were being loaded as the correct types, and that they contained the correct build files. Could they be re-worked to check that more reliably? (They way they're written now is certainly pretty clunky, and the force-unwraps likely just unnecessary given how tests work today.) Or are the PIFLoadingTests generally not very useful at this point?

[owenv]

I couldn't figure out the intent here, because we're comparing BuildFiles to FileReferences, which doesn't seem like it should ever work

[jakepetroules]

Yeah, this definitely seems like it was always broken.

[mhrawdon]

After discussing with Owen, I'm going to take a crack at resolving this in a different way. I may do a pass through this file (which is one of the oldest files in the code base) to make it more robust (e.g., to eliminate force-unwraps).

[mhrawdon]

I think this PR should fix these issues (and clean up a bunch of code besides).