Drop performance tests

[dprotaso]

mako was a Google tool but it's no longer hosted and maintained so figure we can remove it for now

Part of knative/pkg#2820

[knative-prow]

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[ikavgo]

/lgtm

[evankanderson]

Sorry, just catching this now, but I don't think this was using mako at all? It looks like it was using gnuplot and some manual steps.

It would be nice to bring these back in a cross-Eventing way in some form, rather than having them buried in the rabbitmq implementation, but I wanted to make sure that I understood whether these were using Mako at all or not.

[gabo1208]

We were using mako as a sidecar to knative perf test images, so maybe we didn't need mako at all, or just for some aggregation.

Should be not that hard to reuse the same images and aggregate, and just automating something like this results script