functionObjects: Simplified the handling of the post-processing mode

Replaced the 'postProcess' argument to the 'write' and 'execute'
functions with the single static member 'postProcess' in the
functionObject base-class.

src/OpenFOAM/db/functionObjects/functionObjectList/functionObjectList.H

@@ -235,7 +235,7 @@ public:
         //- Called at each ++ or += of the time-loop.
         //  postProcess overrides the usual executeControl behaviour and
         //  forces execution (used in post-processing mode)
-        bool execute(const bool postProcess = false);
+        bool execute();
 
         //- Called when Time::run() determines that the time-loop exits
         bool end();