[C++] There can be only one stdplane

Fixes: https://github.com/dankamongmen/notcurses/issues/409

Standard planes can be duplicated but the copy must not be treated as a
standard plane. There can be only one :)

Make it so.

include/ncpp/Plane.hh

@@ -985,7 +985,6 @@ namespace ncpp
 			if (ret == nullptr)
 				throw init_error ("notcurses failed to duplicate plane");
 
-			is_stdplane = other.is_stdplane;
 			return ret;
 		}