suzerain: eliminate spurious fsolver_set

These calls were being made with more resiliency inside
the static fsolver_solve method.  When done here, they
cause a gsl_error which fsolver_solve protects against.

git-svn-id: https://svn.ices.utexas.edu/repos/pecos/suzerain/trunk@43041 0bd85f0b-c151-43b9-a1e9-27231104551f