chore: remove unused cleanup code

[robsimmons]

This code wasn't running anyway: bwrap's --die-with-parent option (plus --unshare-pid) means that the collab server got SIGKILLed.

(Additionally, this may not have been doing what we expected even if it had run, because it was only persisting documents that someone had open in their vscode editor: but we terminated collaboration servers after terminating vscode editor sessions, so from the collaboration server's perspective, there were no vscode sessions with tabs open.)

[Vtec234]

I don't think this is the right change; the SIGKILL issue (#62) should be fixed, and we should be sending SIGTERM. We do want to stop saving documents to disk - I made that change in #160 - but we should keep server.destroy(); db.close() to persist collab.db.