[TMCU-989] Post-prod cleanup: remove PureBlackProvider and usePureBlack hook from design-system-react

### Description\n\nMirrors Jira [TMCU-989](https://consensyssoftware.atlassian.net/browse/TMCU-989).\n\nPart of epic #1271 / [TMCU-622](https://consensyssoftware.atlassian.net/browse/TMCU-622).\n\n**Jira status:** To Do\n\n### Jira details\n\n## Context\n\n and were provisional scaffolding to let extension and mobile opt into the pure-black token set before it was merged into . Once is updated, the provider pattern is no longer needed \u2014 consumers simply use directly without any wrapper.\n\n## Files to remove / change\n\n* \u2014 remove entire directory\n* \u2014 remove PureBlackProvider and usePureBlack exports\n* :\n\n * Remove import and its usage in \n * Remove globalType and helper\n * Simplify component to not branch on \n \n\n## Acceptance criteria\n\n* and are not exported from \n* Storybook preview no longer shows the Pure Black toolbar toggle\n* No remaining attribute handling in the preview decorator\n* This is a breaking change \u2014 requires a major version bump on \n\n## Dependencies\n\n* Must land together with or after removal from shared\n\n\n