Remove one-time survey code

[Samwalton9]

Description

This PR removes code we wrote for a one-time user survey sent a year ago. We have no near-term plans to send more of these, so I set out primarily to remove the db column and the localisable strings, along with any code associated to this one-time survey. I didn't remove everything, though, and left the email backend and env variables because they were generic and could be used for future mediawiki API emails.

See https://github.com/WikipediaLibrary/TWLight/pull/1497/changes and https://github.com/WikipediaLibrary/TWLight/pull/1498/changes for the direct comparison of what was added.

Definitely open to the idea of removing more or less than I have here, I wasn't 100% sure I got the balance right.

Rationale

Removes unused code; drops some unnecessary data; reduces translator burden.

Phabricator Ticket

https://phabricator.wikimedia.org/T441242

How Has This Been Tested?

Ran tests; clicked around site locally and verified nothing appeared broken.

Types of changes

What types of changes does your code introduce? Add an x in all the boxes that apply:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Minor change (fix a typo, add a translation tag, add section to README, etc.)