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/Multiplayer/TestSceneMultiplayerMatchSongSelect.cs

@@ -83,7 +83,7 @@ namespace osu.Game.Tests.Visual.Multiplayer
                 beatmapSetInfo.Beatmaps.Add(beatmap);
             }
 
-            manager.Import(beatmapSetInfo).WaitSafely();
+            manager.Import(beatmapSetInfo);
         }
 
         public override void SetUpSteps()