[multidomain][assembly] deactivate multithreaded assembly for fcstaggered

What this MR does / why does DuMux need it:

There seems to be a race condition in multithreaded assembly for freeflow problems using a couplingmanager. Now it has been observed that also with the fcstaggered discretization simulation results can be inconsistent between runs when using multi-threaded assembly. This MR temporarily disables multithreaded assembly for the free-flow fcstaggered couplingmanager until the race condition has been found and resolved.