reduce template file size

As reported on [the forum](https://forum.getodk.org/t/easily-reduce-the-file-size-of-the-google-sheet-template-xlsform-in-the-docs/56795/2), the .xlsx file size is about 500KB which is quite high for a mostly empty workbook. It seems to be due to cell styling applied to all rows/columns.

Start by locating the .xlsx template and inspecting its cell styling, using the linked forum report as context. Reduce unnecessary styling while preserving the workbook's existing behavior, then compare the resulting file size and confirm the template still opens and remains usable.

Written by the indexing model from the issue text.