ActivityTracker is mostly untrimmed even when event source is disabled

[vitek-karas]

ActivityTracker really only works if TplEventSource is enabled, so if all event sources are disabled via a feature switch EventSourceSupport there's really no need to keep its implementation around for anything.

Ideally we would refactor the code in it such that linker will be able to remove most of the implementation.

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[LakshanF]

In addition to the TplEventSource, AssemblyLoadContext and EventSource has references to this class

[eerhardt]

@LakshanF - are you addressing this issue with the EventSource work you've been doing?

[LakshanF]

I was not planning to - I took a quick look when I closed #43657 to see if its an easy fix but noted the additional references from AssemblyLoadCpntext, EventSource and left this issue separate from that work