Summary:
Now that we have a way to write cost issues, let's not rely on some arbitrary threshold (and also get rid of `EXPENSIVE_EXECUTION_TIME` issues in tests).
One consequence of this is that we will loose the cost traces in tests since `costs-report.json` doesn't have any traces. Next diff fixes that.
Reviewed By: skcho
Differential Revision: D21837574
fbshipit-source-id: 86b4d028d