You can delete a placeholder like any other object by selecting it and then pressing the Del key. Alternatively, you can right-click on the placeholder to open the context menu and select Delete here.
If you choose the ribbon command Design | group Placeholders | Restore 
, all placeholders that you have deleted in the current slide will be restored with their original properties (size, position, formatting).
This command is available only if placeholders were actually deleted from the slide.
For more information, see also Resetting slides.