Revert "[analyzer] Do part of the work to find shortest bug paths up front."

The whole reason we were doing a BFS in the first place is because an
ExplodedGraph can have cycles. Unfortunately, my removeErrorNode "update"
doesn't work at all if there are cycles.

I'd still like to be able to avoid doing the BFS every time, but I'll come
back to it later.

This reverts r177353 / 481fa5071c203bc8ba4f88d929780f8d0f8837ba.
[analyzer] Add a test case for diagnostic suppression on a graph with cycles.

(see previous commit)