Remove Task from the inner-most Import method in RealmArchiveModelImporter

One of my pending work items for post-realm merge.

The lowest-level import task is no longer asynchronous, as we don't want
it to span multiple threads to allow easier interaction with realm.
Removing the `Task` spec simplifies a heap of usages.

Individual usages should decide whether they want to run the import
asynchronously, by either using an alternative override or spooling up a
thread themselves.

osu.Game.Tests/Visual/SongSelect/TestSceneBeatmapLeaderboard.cs

@@ -180,7 +180,7 @@ namespace osu.Game.Tests.Visual.SongSelect
             AddStep(@"Load new scores via manager", () =>
             {
                 foreach (var score in generateSampleScores(beatmapInfo()))
-                    scoreManager.Import(score).WaitSafely();
+                    scoreManager.Import(score);
             });
         }