chore: Removes DeprecatedLoading, DeprecatedModal and Checkbox

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Short description

These 3 components are no longer used in either Todoist or Twist, so I'm taking the opportunity that we'll have a breaking change release soon (see #671) to remove them entirely.

Test plan

• Double-check that Todoist and Twist codebase do not use any of these components.

  For the two with the word "deprecated" in their names, it is straightforward.

  For Checkbox is trickier, but make sure to search in the entire codebase that name with case-sensitive search and the exact word. Then exclude the test files, cypress, i18n files, to get less false positives. You should be left with a relatively small number of hits that you can then validate are non from importing from Reactist.

PR Checklist

• Added tests for bugs / new features
• Updated docs (storybooks, readme)
• Executed npm run validate and made sure no errors / warnings were shown
• Described changes in CHANGELOG.md
• Bumped version in package.json and package-lock.json (npm --no-git-tag-version version <major|minor|patch>) ref
• Updated all static build artifacts (npm run build-all)

Versioning

New major release.