This one is nearly intractable -- the underlying text handles are tied to the color of the line, although one could set their color independently by use of the CData property for each.
BUT, the kicker is, internally, the handles to those text objects comes and goes depending upon whether the 'LineStyle' or 'LineColor' property are set -- if either of those is set to 'none' to hide the lines, then the text objects themselves disappear entirely, they aren't just hidden.
So (and I didn't have time just now to try to do it), the only way I see to go at it would be to
- create the plot; save the handle to the contour object [~,hC]=contourf(...);
- make sure lines/contour levels are showing
- save array of text object handles from undocumented TextPrims property -- hTxt=hC.TextPrims;
- retrieve all the poop about each -- including 'VertexData', 'Rotation', 'String', etc., etc., ...
- turn off 'LineStyle' by hC.LineStyle='none';
- now redraw all the text objects from the data saved in step 4) above.
Without such machinations, the next best thing to no line might be
to used the dotted line as subtly as possible. Unfortunately, HG internals is such that even 'LineWidth' of eps is still rendered as if were 0.5 and identically zero isn't allowed (and if were, the above behavior of deleting the text objects would also probably happen, anyways).