Remove the `secrets` table entirely

Created by: flying-robot

This table is unused and contains no records, so it can be safely removed.

dev-readonly@sg=> select * from secrets;
┌────┬─────────────┬───────────┬──────────┬───────┐
│ id │ source_type │ source_id │ key_name │ value │
├────┼─────────────┼───────────┼──────────┼───────┤
└────┴─────────────┴───────────┴──────────┴───────┘
(0 rows)

Time: 51.165 ms

ref https://sourcegraph.slack.com/archives/CHPC7UX16/p1611929489096700