Fix the PD algo factory

Author: Michael McLeod

cpp/purify/algorithm_factory.h

@@ -273,7 +273,6 @@ primaldual_factory(
       .Phi(*measurements)
       .tau(0.5 / (measurements->sq_norm() + 1))
       .xi(1.)
-      .nu(op_norm * op_norm)
       .sigma(1.);
 #ifdef PURIFY_MPI
   ConvergenceType obj_conv = ConvergenceType::mpi_global;