Remove deprecated control field from handler form

Problem/Motivation

When creating a webform mailchimp handler the form contains a deprecated field 'control'. To prevent confusion for the user I would like to remove this field from the from since it is not used.

Steps to reproduce

Create a Mailchimp handler for a webform
Verify that the deprecated control field is shown in the handler form

Proposed resolution

Remove the control field from the handler form

Remaining tasks

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User interface changes

Remove the control field from the handler form

API changes

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Data model changes

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