Doesn't seem to be any way to prevent the builtin fit object plot() routine from adding the legend.
Instead, use
plot(x,myfit(x),'linespec');
for the x that was the input to create myfit object. This will bypass the cfit object plot by evaluating it only and then call the base MATLAB plot() routine.
Does seem worthy of enhancement request to allow for further customization options on creation of fit object plots.
I didn't try to do comparison timings...