Revert "Add command signal handler."

[namespacebrian]

Reverts #2685

[namespacebrian]

Reason for the revert: this signal handler assumes direct responsibility for what shutdown/clean-up tasks need to be done. I had envisioned something more adaptive/generalized where the signal handler sets a flag, or invokes more callbacks/hooks that let other parts of the code do their own clean-up tasks.

My reasoning is that keeping it in now runs the risk of more cleanup tasks being added to it, increasing its responsibilities and the amount of contexts it attempts to address.. conflation of concerns.

If what I had envisioned is infeasible, or we conclude that having this is better than nothing, we can revert this revert and easily put it back, but I'd like to pause at least for now.