The reason you cannot tab through the legacy form is that it is not protected for forms. Legacy forms must be protected for forms.
With that protection in place you could use something like the following to delete the checkbox if it is unchecked.
Dim bProtected As Boolean
If ActiveDocument.FormFields("deckscope2c").CheckBox.Value = False Then
'Unprotect the file
If ActiveDocument.ProtectionType <> wdNoProtection Then
bProtected = True
ActiveDocument.Unprotect Password:=""
End If
ActiveDocument.FormFields("deckscope2c").Range.Delete
'Reprotect the document.
If bProtected = True Then
ActiveDocument.Protect _
Type:=wdAllowOnlyFormFields, _
NoReset:=True, _
Password:=""
End If
End If
If you have changed the form fields to content controls and have titled the content controls then to delete the "deckscope2c" content control if it is unchecked
Dim oCC As ContentControl
Dim oRng As Range
For Each oCC In ActiveDocument.ContentControls
If oCC.Title = "deckscope2c" Then
If oCC.Checked = False Then
Set oRng = oCC.Range
oCC.Delete
oRng.Delete
End If
End If
Next oCC