[TMCU-988] Post-prod cleanup: remove PureBlackContext and PureBlackProviderProps from design-system-shared public API

### Description\n\nMirrors Jira [TMCU-988](https://consensyssoftware.atlassian.net/browse/TMCU-988).\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 added to as a shared contract for the provisional pure-black provider pattern. Once absorbs the pure-black values and the provider is removed, these are dead API surface on a shared package consumed by both extension and mobile.\n\n## Files to remove / change\n\n* \u2014 remove entire directory\n* \u2014 remove the PureBlack context export block\n* Verify no remaining imports of or anywhere in the monorepo\n\n## Acceptance criteria\n\n* , , and are not exported from \n* No remaining references to these symbols in the monorepo\n* This is a breaking change \u2014 requires a major version bump on \n\n## Dependencies\n\n* Must land after the promotion (TMCU-987)\n* Must land together with or after removal from \n\n\n