Remove unencrypted webhook token and URL columns

What does this MR do?

In https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/21645 , we added encrypted_token and encrypted_url columns to the web_hooks table in the database, along with a background migration that moved all data from the unencrypted columns to the encrypted ones.

In this MR, we remove the now-unpopulated-and-unused columns, first ensuring that the background migration has completed.

What are the relevant issue numbers?

Does this MR meet the acceptance criteria?

• [-] Changelog entry added, if necessary
• [-] Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides

Closes #52367 (closed)