With this xla file open, go into the VBE (Visual Basic Editor) using Alt+F11 and locate this file in the top left Project Window.
Find the ThisWorkbook module within this workbook in the Project Window and double-click it to open it.
In the properties window (probably on the bottom left of the VBE under the Project Window), in the list of properties set the IsAddin property to False.
Back in Excel, the xla will now be visible. Select it and click File>SaveAs to save down as a non-xla file.