Remove dead code (part 1)

[mathemancer]

Related to #3946

Removes dead code from the mathesar library.

Technical details

Main parts removed are:

• API tests which aren't relevant for the new architecture
• unused paths (in urls.py)
• Unused viewsets
• Unused serializers
• Unused API exception classes
• Dependencies of the above code (which aren't otherwise used)

This PR does not remove:

• Any models, used or not
• Any code that supports models
• Any code from the db library.

Checklist

• My pull request has a descriptive title (not a vague title like Update index.md).
• My pull request targets the develop branch of the repository
• My commit messages follow best practices.
• My code follows the established code style of the repository.
• I added tests for the changes I made (if applicable).
• I added or updated documentation (if applicable).
• I tried running the project locally and verified that there are no
  visible errors.

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[mathemancer]

Please do not merge this until we cut the 0.2.0-testing.1 branch!

[mathemancer]

@Anish9901 This is ready for review, but (again) don't merge until we cut the release branch.

[Anish9901]

Rest in git history!

[Anish9901]

I think we can completely remove this file in your subsequent PR, I don't see is_superuser and is_self used anywhere.

They're used in mathesar.api.ui.permissions.users.UserAccessPolicy via 🪄magic🪄 .

[Anish9901]

I would like to have this file around in our codebase, I frequently use it to test import as well as test type inference manually. @mathemancer can you consider restoring this file in your subsequent PR?

[mathemancer]

I see your point, but I'd like to get everything completely out first, then add things back in that we still want later. In that example, we'd want to have a separate directory where "human testing" data or files would reside.

I suggest making an issue for adding "human testing" data to the repo where we can discuss further. (maybe we'd actually use the mathesar-data-playground repo or something)