Cleanup unused deprecated native checkbox element

What does this MR do and why?

Detected usage of native checkbox usage inside HAML file that must be migrated to Pajamas::CheckboxComponent.

It's a part of Pajamas adoption project

UPDATE: After some discussions we decided to remove the file with checkbox component as not used.

I see we added this checkbox here cca4a5fd and it was rendered only from next files:

• app/views/shared/boards/components/sidebar/_labels.html.haml
• app/views/shared/issuable/_board_create_list_dropdown.html.haml

according to passed , locals: { show_add_list: true } while rendering = render partial: "shared/issuable/label_page_create", locals: { show_add_list: true }

Later we removed both files:

• app/views/shared/boards/components/sidebar/_labels.html.haml here 58f5d646
• app/views/shared/issuable/_board_create_list_dropdown.html.haml here !59826 (diffs)

I don't see we pass this , locals: { show_add_list: true } anywhere else and it's false by default so most likely this checkbox is not used anymore and can be removed.

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Related to #446219 (closed)