Fix social link form resubmission after removing invalid link (#6945)

Fix social link form resubmission after removing invalid link (#6945)

When a social link with validation errors is removed via the "Remove"
button, the row is hidden with d-none but the input retains its
required attribute. This prevents form submission because HTML5
validation rejects hidden required fields that cannot be focused.

Strip the required attribute from the text input when marking a social
link for destruction so the form can be resubmitted.

Closes #6903