Remove level_source fixture

[Hamms]

This one-and-a-half megabyte yaml file has been causing some issues when loading level fixtures, and looks to be set to cause even more issues on Rails 6.

Specifically, when trying to load fixtures as a part of rake db:test:prepare, we get the error Mysql2::Error: Commands out of sync; you can't run this command now. It seems to be related to an issue with Mysql's max_allowed_packet configuration, We previously tried to bypass this by increasing the setting on the client (#38123) but it looks like to actually fix it we also need to increase the setting on the server, which is of course a bit more complicated.

Instead, I'd simply like to do away with this enormous fixture. Looks like we have a total of three tests which are relying on it; two of which could simply be using FactoryBot instead to create the data they need, the third of which seems to just be testing that fixtures are getting loaded and so (unless I'm fundamentally misunderstanding the purpose of the test) is not testing anything that we actually care about.

Follow-up work

We've made a couple of attempts to adjust the max_allowed_packet value; once we've merged this, we can go through and undo those since this should render them unnecessary.

Reviewer Checklist:

• Tests provide adequate coverage
• Privacy and Security impacts have been assessed
• Code is well-commented
• New features are translatable or updates will not break translations
• Relevant documentation has been added or updated
• User impact is well-understood and desirable
• Pull Request is labeled appropriately
• Follow-up work items (including potential tech debt) are tracked and linked

[davidsbailey]

Whoa, nice work Elijah! We are 100% on board with moving off test fixtures. This one looks like it was easier to remove than I feared. I just ran into the max packet problem recently on my local environment. This seems like it will help a lot!

[Hamms]

Haha, yeah I'm not gonna lie I was pretty shocked to see a green checkmark on this after fixing just those three tests