#147 draw: explicit() needs no dummy variable when plotting an function

[Gunter Königsmann]

Besides

draw2d(explicit(sin,x,1,10));

the following should be legal:

draw2d(explicit(sin,1,10));

We don't really need a dummy variable in this case.

Originally reported by Stavros Macrakis:
https://sourceforge.net/p/maxima/mailman/message/36299796/