remove Worker#ensure_db_connection

test plan: run a delayed jobs pool, and verify that jobs still complete successfully

Change-Id: I9fbadee51b05f667dafcb9173af2e75bf905c1e0
Reviewed-on: https://gerrit.instructure.com/40453
Reviewed-by: Cody Cutrer <cody@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Jeremy Putnam <jeremyp@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>