Remove RUSTUP_TRACE_DIR unstable feature

[sanmai-NL]

Problem you are trying to solve

https://rust-lang.github.io/rustup/environment-variables.html lists this feature. But the Catapult tracing tool it requires is long archived (for 4 years). As the feature is still unstable, perhaps remove it?

I don't know what tracing is done but logging and OS-afforded tracing can be done on rustup. Does that cover its original use case?

Solution you'd like

Remove unstable, derelict features to increase maintainability.

Notes

No response