STYLE: prefer REGISTER/NO_REGISTER instead of true/false for IOobject

- self-documenting

applications/utilities/preProcessing/mapFields/mapFields.C

@@ -67,7 +67,7 @@ int readNumProcs
                     runTime,
                     IOobject::MUST_READ,
                     IOobject::NO_WRITE,
-                    false // do not register
+                    IOobject::NO_REGISTER
                 ),
                 args.getOrDefault<fileName>(optionName, "")
             )
@@ -360,7 +360,7 @@ int main(int argc, char *argv[])
                 runTimeTarget,
                 IOobject::MUST_READ_IF_MODIFIED,
                 IOobject::NO_WRITE,
-                false
+                IOobject::NO_REGISTER
             )
         );