With Bug 1668219, we removed the old performance panel for good. Before this, we had a way to switch between old/new with a preference. But now it's gone, we can remove all of the instrumentation code, related to markers etc.
Here are some examples (not exhaustive):
recordProfileTimelineMarkers
Probably the whole TimelineConsumers? (not sure if it's used outside of the old performance panel. we neeed to look, but I highly doubt it.)
But if we can't remove everything there, at least we should remove:
TimelineConsumers::AddMarkerForDocShell, TimelineConsumers::AddMarkerForAllObservedDocShells TimelineConsumers::PopMarkersand their usages.

Also we have a bunch of marker classes here that we can remove: https://searchfox.org/mozilla-central/search?q=%3A+public+TimelineMarker&path=&case=false&regexp=false

And we have some tests we can remove: https://searchfox.org/mozilla-central/search?q=&path=docshell%2F**test%2F**timeline&case=false&regexp=false
And we have the testing functions in the head.js and frame-head.js files:
https://searchfox.org/mozilla-central/rev/5f10809bf5559e6e988e4d1a58ce1338d391cc5f/docshell/test/browser/frame-head.js#56-114
https://searchfox.org/mozilla-central/rev/5f10809bf5559e6e988e4d1a58ce1338d391cc5f/docshell/test/browser/head.js#11-258