API: Remove IS_WPCOM conditional before wpcom_json_api_request_as_blog

Now that `wpcom_json_api_request_as_blog` contains an abstraction to call the WP.com API directly when running on WP.com we no longer need to add `IS_WPCOM` conditionals before calling `wpcom_json_api_request_as_blog`. Instead we can simply remove all of the `IS_WPCOM` code block and call `wpcom_json_api_request_as_blog` in both environments.

I have managed to find 4 instances in the code where this needs updating:

- [ ] [class-jetpack-instagram-gallery-helper.php](https://github.com/Automattic/jetpack/blob/a1fe6f0b03ea68fb5e0eed1e22d585e2351b2a06/_inc/lib/class-jetpack-instagram-gallery-helper.php#L29)
- [ ] [class-wpcom-rest-api-v2-endpoint-mailchimp.php ](https://github.com/Automattic/jetpack/blob/092d9af6c96384083ae509741be77a8db9346721/_inc/lib/core-api/wpcom-endpoints/class-wpcom-rest-api-v2-endpoint-mailchimp.php#L118)
- [ ] [class-jetpack-instagram-widget.php](https://github.com/Automattic/jetpack/blob/8ab0362810c20f6dec9f269d0d4fa3aa3dc82f0e/modules/widgets/class-jetpack-instagram-widget.php#L171)
- [ ] [class-jetpack-tweetstorm-helper.php](https://github.com/Automattic/jetpack/blob/9a437395f0d2f8248bea436faacd8445e71de846/_inc/lib/class-jetpack-tweetstorm-helper.php#L231)

See p7H4VZ-2LY-p2 and #15468 for details.

Review the four named files: class-jetpack-instagram-gallery-helper.php, class-wpcom-rest-api-v2-endpoint-mailchimp.php, class-jetpack-instagram-widget.php, and class-jetpack-tweetstorm-helper.php. In each location, remove the IS_WPCOM conditional around wpcom_json_api_request_as_blog and call it in both environments. Done means all four instances use the shared abstraction without the conditional blocks.

Written by the indexing model from the issue text.