functionObjectList: Rationalized and simplified the handling of disabled functionObjects

Simplified and generalized the handling of functionObjects which fail to
construct by removing them from the list rather than maintaining an
"enabled" switch in each functionObject.

src/OpenFOAM/db/Time/Time.C

@@ -699,7 +699,7 @@ Foam::Time::~Time()
         removeWatch(controlDict_.watchIndex());
     }
 
-    // destroy function objects first
+    // Destroy function objects first
     functionObjects_.clear();
 }