[BE] Make visible the checkbox field in the sidebar

• Type: Technical Task
• Status: Closed
• Priority: Minor
• Resolution: Completed
• Affects Version/s: None
• Fix Version/s: 7.4.13 DXP GA1, Master
• Component/s: Forms
• Labels: None
• Sprint: Forms_7.4_19, Forms_7.4_20
• Git Pull Request: https://github.com/liferay-forms/liferay-portal/pull/803

• CheckboxDDMFormFieldType.java and remove the property: "ddm.form.field.type.system=true"