[analyzer][NFC] Refactoring BugReporter.cpp P6.: Completely get rid of interestingness propagation

Apparently this does literally nothing.

When you think about this, it makes sense. If something is really important,
we're tracking it anyways, and that system is sophisticated enough to mark
actually interesting statements as such. I wouldn't say that it's even likely
that subexpressions are also interesting (array[10 - x + x]), so I guess even
if this produced any effects, its probably undesirable.

Differential Revision: https://reviews.llvm.org/D65487

llvm-svn: 368752