simplify code by appling varargs messages and warnings

src/update.cpp

@@ -305,9 +305,8 @@ void Update::set_units(const char *style)
 
   // check if timestep was changed from default value
   if (!dt_default && (comm->me == 0)) {
-    error->warning(FLERR,fmt::format("Changing timestep from {:.6} to {:.6} "
-                                     "due to changing units to {}",
-                                     dt_old, dt, unit_style));
+    error->warning(FLERR,"Changing timestep from {:.6} to {:.6} due to "
+                   "changing units to {}", dt_old, dt, unit_style);
   }
   dt_default = 1;
 }