don't clear the party sheet when clearing scenario graphics.

This is because when you leave a scenario and enter a new one,
your party sheet gets clobbered

src/gfx/gfxsheets.hpp

@@ -30,7 +30,6 @@ struct cCustomGraphics {
 	bool is_old = false;
 	void clear() {
 		sheets.clear();
-		party_sheet.reset();
 	}
 	explicit operator bool() {
 		return !sheets.empty() && bool(sheets[0]);