Avoid unnecessary expansion in sqrfree_yun().

Avoid unnecessary expansion in sqrfree_yun().

Don't make the polynomial primitive inside Yun's algorithm. Computing the
primitive part will incur an extra expansion of the polynomial and this
may distroy a pre-factored structure in the other variables. In the multi-
variate case, working with primitive polynomials isn't sufficient anyways
to avoid a final division to discover lost factors.