Looking at your document with the Selection Pane open, it is a mess! Someone went to a lot of trouble to insert that border. It is made up of multiple grouped shapes.
The following macro will remove all shapes in your document. It gets rid of the line.
Use it on a copy because it may remove things you do not realize are shapes.
Code:
Sub ShapesDelete()
Dim oShape As Shape
' MsgBox ActiveDocument.Shapes.Count
Do Until ActiveDocument.Shapes.Count = 0
For Each oShape In ActiveDocument.Shapes
oShape.Delete
Next oShape
Loop
End Sub
Instructions for Installing Macros from Forums or Websites by Graham Mayor, MVP