[keyserver] remove unnecessary inviteResponder line in keyserver.js

Summary:
We're planning on running the webapp, keyserver, and landing page in isolation in separate containers for our prod self-host setup.

This means we should remove this line server.get('/invite/:secret', inviteResponder);

Test Plan: flow check. I believe we plan on testing this on prod and check if the invite links still works. We can fully remove if confirmed working

Reviewers: ashoat, tomek

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D12808