Remove GuardedAsyncTask wrapper for call.cancel() (#48251)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/48251

The original comment mentioning https://github.com/square/okhttp/issues/869 predates the [upgrade to okhttp3](https://github.com/facebook/react-native/commit/6bbaff2944dafd6fa7e5b77ef46dece0ec2c9983), which resolved the issue via https://github.com/square/okhttp/issues/1592.

Cancel calls should now be async and don't need to be guarded.  Removing this also fixes a discrepancy in NetworkingModule unit test verification.

Changelog: [Internal]

Reviewed By: javache

Differential Revision: D67157018

fbshipit-source-id: 72ce4deaaff306ef1dde6eda7be88707c37f0be7