Remove references to topics in structure.sql
Remove references to topics in structure.sql

A change to structure.sql was committed by accident [1] and it added some DB
structure changes from a WIP branch. This commit undoes the error.

[1] - See commit 5e929e82407268a25fb8055bd57b1f4b5de18863

alphagov/service-manual-publisher,alphagov/service-manual-publisher,alphagov/service-manual-publisher