chore: remove redundant data-schema unit tests

[iartemiev]

Description of changes

• Migrates unit tests that verify model-based generateClient functionality from this repo into api-next
• Retains minimal coverage for:
  1. basic happy path crud/subscribe for standard and server clients
  2. a few edges that are difficult to test from api-next (e.g. Amplify config-based headers)
• DRYs up remaining tests
• Removes unused fixtures, e.g. gen2 schema, MIS, etc.

Description of how you validated changes

• y test succeeds, coverage % unaffected

Checklist

• PR description included
• yarn test passes
• Unit Tests are changed or added
• Relevant documentation is changed or added (and PR referenced)

Checklist for repo maintainers

• Verify E2E tests for existing workflows are working as expected or add E2E tests for newly added workflows
• New source file paths included in this PR have been added to CODEOWNERS, if appropriate

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.