Fix bug in data migration: don't call "Repo.Update"

It's not needed, and it actually doesn't work as expected.

--HG--
branch : dev

src/Orchard.Tests/DataMigration/DataMigrationTests.cs

@@ -294,7 +294,7 @@ features:
             Assert.That(_repository.Table.First().Version, Is.EqualTo(666));
         }
 
-        [Test, Ignore("Fix pending")]
+        [Test]
         public void SameMigrationClassCanEvolve() {
             Init(new[] { typeof(DataMigrationSameMigrationClassCanEvolve) });
 

src/Orchard/Data/Migration/DataMigrationManager.cs

@@ -145,7 +145,6 @@ namespace Orchard.Data.Migration {
                 }
                 else {
                     dataMigrationRecord.Version = current;
-                    _dataMigrationRepository.Update(dataMigrationRecord);
                 }
             }
         }