Fix double-tagging in Prefetches, improved SessionTagPrefetchEntitySetTest

Author: SergeiPavlov

Orm/Xtensive.Orm.Tests/Linq/TagTest.cs

@@ -888,6 +888,7 @@ public void SessionTagPrefetchEntitySetTest()
 
         Assert.That(allCommands.Count, Is.EqualTo(2));
         Assert.IsTrue(allCommands[1].StartsWith("/*outermost*/"));
+        Assert.AreEqual(Regex.Matches(allCommands[0], "outermost").Count, 1);
 
         allCommands.Clear();
       }

Orm/Xtensive.Orm/Orm/Internals/Prefetch/EntitySetTask.cs

@@ -171,11 +171,6 @@ private QueryTask CreateQueryTask()
       var session = manager.Owner.Session;
       var scope = new CompiledQueryProcessingScope(null, null, parameterContext, false);
       QueryProvider = (CompilableProvider) session.StorageNode.InternalQueryCache.GetOrAdd(key, generator);
-      if (session.Tags != null) {
-        foreach (var tag in session.Tags) {
-          QueryProvider = new TagProvider(QueryProvider, tag);
-        }
-      }
       ExecutableProvider executableProvider;
       using (scope.Enter()) {
         executableProvider = session.Compile(QueryProvider);