Tools - Marking Cells - Unmark All

Ribbon bar. Select the Tools tab. Click Marked Cells and from the menu, select Unmark All...

Classic menus. From the Tools - Marking Cells submenu, select Unmark All (or click the toolbar button located on the Tools toolbar)...

...to unmark all of the cells in the spreadsheet (i.e., the Marked Cells spreadsheet layout will be removed from all cells). See also: Format - Overview, Spreadsheet Layouts - Overview.