This was removed in rB0666ece2e2f9 because it is handled differently for
"real" cloth in cloth_solve_collisions(), but hair still needs this
apparently [does its thing in cloth_continuum_step()] instead.
And since we have a default 'Quality Steps' setting of 5, it made many
simulations instable [in fact I am still going over reports that might
be affected by this...]