[dagit] Delete some dead code
Run yarn find-dead-code to find unused modules in @dagit/core, now that I've removed a couple routes that we aren't using.
Also consolidated some of the instance status page components for sensors/schedules, since we can do that now.
Test Plan: yarn find-dead-code, lint, ts.
Reviewers: bengotow, prha
Reviewed By: bengotow
Differential Revision: https://dagster.phacility.com/D7505