Remove mention of nonexistent DELETE /scheduler/jobs

The README mentions an endpoint for deleting all jobs, but the endpoint
does not exist. Requesting "/scheduler/jobs" with DELETE returns an
error. The endpoint is not exactly desirable, and the README even said,
"Note: *don't do this*."

Fixes #158
Merge pull request #5 from mesosphere/remove-dead-endpoint-from-readme

Remove mention of nonexistent DELETE /scheduler/jobs