chore(showcase/integrations): drop final stale null overrides for now-canonical features

5 cells unblocked by removing per-framework null overrides that were
overriding the canonical defaults landing in the companion commit:

- google-adk: drop nulls for voice, byoc-hashbrown, byoc-json-render
  (canonicals now wired in feature-registry).
- langgraph-python: drop nulls for voice, byoc-hashbrown,
  byoc-json-render (same).

After this commit, no framework has a stale `null` override for any
documented feature. The remaining intentional opt-outs (subagents on 4
fws) keep their existing nulls because they're genuinely framework-
specific decisions, not stale config.

Author: samjulien

showcase/integrations/google-adk/docs-links.json

@@ -108,18 +108,6 @@
     "readonly-state-agent-context": {
       "og_docs_url": "https://docs.copilotkit.ai/adk/shared-state/in-app-agent-read",
       "shell_docs_path": "/shared-state/agent-readonly"
-    },
-    "voice": {
-      "og_docs_url": null,
-      "shell_docs_path": null
-    },
-    "byoc-json-render": {
-      "og_docs_url": null,
-      "shell_docs_path": null
-    },
-    "byoc-hashbrown": {
-      "og_docs_url": null,
-      "shell_docs_path": null
     }
   }
 }

showcase/integrations/langgraph-python/docs-links.json

@@ -129,21 +129,9 @@
       "og_docs_url": "https://docs.copilotkit.ai/langgraph/multi-agent-flows",
       "shell_docs_path": "/multi-agent-flows"
     },
-    "voice": {
-      "og_docs_url": null,
-      "shell_docs_path": null
-    },
     "auth": {
       "og_docs_url": "https://docs.copilotkit.ai/langgraph/auth",
       "shell_docs_path": "/auth"
-    },
-    "byoc-hashbrown": {
-      "og_docs_url": null,
-      "shell_docs_path": null
-    },
-    "byoc-json-render": {
-      "og_docs_url": null,
-      "shell_docs_path": null
     }
   }
 }