TaskManager: Remove the explicit use of Core::Id::toUniqueIdentifier()

Directly comparing the Ids gives the same effect of a stable
(but unpredictable) sort.

Change-Id: Iaf54a127b02f2a89eeff9b6d008575bb899ef926
Reviewed-by: Daniel Teske <daniel.teske@theqtcompany.com>

src/plugins/projectexplorer/task.cpp

@@ -137,9 +137,9 @@ bool operator<(const Task &a, const Task &b)
         // Can't happen
         return true;
     } else {
-        if (a.category.uniqueIdentifier() < b.category.uniqueIdentifier())
+        if (a.category < b.category)
             return true;
-        if (b.category.uniqueIdentifier() < a.category.uniqueIdentifier())
+        if (b.category < a.category)
             return false;
         return a.taskId < b.taskId;
     }