Unsubscribe from all lists when use selects "None"

Problem/Motivation

I couldn't understand how to allow users to unsubscribe. The documentation and code mention deleting the field, but I am unaware of how a user can delete a field from their profile/account without deleting the entire account.

Proposed resolution

When a user selects "None" from the available options, they will be unsubscribed from any and all lists.

Remaining tasks

My patch works functionally. However, there are "RequestException: " errors logged for each list entry upon unsubscribe.

For example, if your Moosend account has 3 lists, when a user unsubscribes, the Drupal log will show an error of "RequestException:" 3 times (once for each list), each followed by a success entry that reads "submitContactForm has been executed successfully."

I've been unable to track down what exactly is triggering the "RequestException:" errors thus far, but functionally, the unsubscribe is working.