Removes DRAKE_DEMANDs temporily added during a PR train transition pe…

…riod. (RobotLocomotion#21469)

We added a number of DRAKE_DEMANDs in SapDriver::AddContactConstraints() during
a transition period in PR RobotLocomotion#20913. In that PR we explicitly loaded NANs to mark
not yet supported configurations (in this case deformables + HuntCrossley
contact constraints). Now that the feature is complete and those configurations
are supported, these DEMANDs can go away.

multibody/plant/sap_driver.cc

@@ -251,16 +251,8 @@ std::vector<RotationMatrix<T>> SapDriver<T>::AddContactConstraints(
             MakeContactConfiguration(pair), std::move(J),
             make_sap_parameters()));
       } else {
-        ContactConfiguration<T> contact_configuration =
-            MakeContactConfiguration(pair);
-        DRAKE_DEMAND(
-            !std::isnan(ExtractDoubleOrThrow(contact_configuration.vn)));
-        DRAKE_DEMAND(
-            !std::isnan(ExtractDoubleOrThrow(contact_configuration.fe)));
-        DRAKE_DEMAND(
-            !std::isnan(ExtractDoubleOrThrow(contact_configuration.phi)));
         problem->AddConstraint(std::make_unique<SapHuntCrossleyConstraint<T>>(
-            std::move(contact_configuration), std::move(J),
+            MakeContactConfiguration(pair), std::move(J),
             make_hunt_crossley_parameters()));
       }
     } else {