db: recreate recent_searches table when migrate down

Created by: unknwon

To avoid noisy error logs in background.

Also removed unnecessary line in *_up.sql, i.e. drop sequence, done automatically by dropping table.

Addresses https://github.com/sourcegraph/sourcegraph/pull/6111/files#r337085913.

Test plan: Locally migrate down and no more error logs complain table not exists.