fix(auth): remove unneeded OAuth flows

This is related to #5327.
`BaseToolset.get_auth_config()` returns `None` by default and removing its overrides in toolsets that don't need OAuth flows to list tools does the job.

No regressions in unit tests (`pytest tests/unittests/auth`):
```
================================================================= 181 passed, 508 warnings in 4.23s =================================================================
```

PiperOrigin-RevId: 915128706

Author: google-genai-bot

src/google/adk/tools/apihub_tool/apihub_toolset.py

@@ -199,13 +199,3 @@ def _prepare_toolset(self) -> None:
   async def close(self):
     if self._openapi_toolset:
       await self._openapi_toolset.close()
-
-  @override
-  def get_auth_config(self) -> Optional[AuthConfig]:
-    """Returns the auth config for this toolset.
-
-    ADK will populate exchanged_auth_credential on this config before calling
-    get_tools(). The toolset can then access the ready-to-use credential via
-    self._auth_config.exchanged_auth_credential.
-    """
-    return self._auth_config

src/google/adk/tools/application_integration_tool/application_integration_toolset.py

@@ -289,13 +289,3 @@ async def get_tools(
   async def close(self) -> None:
     if self._openapi_toolset:
       await self._openapi_toolset.close()
-
-  @override
-  def get_auth_config(self) -> Optional[AuthConfig]:
-    """Returns the auth config for this toolset.
-
-    ADK will populate exchanged_auth_credential on this config before calling
-    get_tools(). The toolset can then access the ready-to-use credential via
-    self._auth_config.exchanged_auth_credential.
-    """
-    return self._auth_config

src/google/adk/tools/openapi_tool/openapi_spec_parser/openapi_toolset.py

@@ -245,14 +245,3 @@ def _parse(self, openapi_spec_dict: Dict[str, Any]) -> List[RestApiTool]:
   @override
   async def close(self):
     pass
-
-  @override
-  def get_auth_config(self) -> Optional[AuthConfig]:
-    """Returns the auth config for this toolset.
-
-    Note: This returns a copy so any exchanged credentials populated by the ADK
-    framework do not persist on the toolset instance across invocations.
-    """
-    return (
-        self._auth_config.model_copy(deep=True) if self._auth_config else None
-    )