Fairly straightforward, just a blanket removal. Haven't heard
anything on dev-platform or fx-data-dev regarding this removal,
so I think it's likely safe to remove on Nightly, and we can
revert if anyone makes a fuss.
As part of removing the HangMonitor, I renamed a few things and
reorganized the namespaces to not depend on a HangMonitor
namespace. Hopefully this doesn't produce too much noise in the
diff, it just seemed appropriate to move everything around
rather than keep dangling vestiges of the old system.
MozReview-Commit-ID: 8C8NFnOP5GU
35bd3f0b0d