[staggered] Remove wrong static assert from c&p mistake

dumux/assembly/staggeredfvassembler.hh

@@ -96,7 +96,6 @@ public:
                  timeLoop,
                  prevSol)
     {
-        static_assert(isImplicit, "Explicit assembler for stationary problem doesn't make sense!");
         this->couplingManager_->setSubProblems(std::make_tuple(problem, problem));
     }