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/UserInterface/TestSceneDeleteLocalScore.cs

@@ -112,7 +112,7 @@ namespace osu.Game.Tests.Visual.UserInterface
                         Ruleset = new OsuRuleset().RulesetInfo,
                     };
 
-                    importedScores.Add(scoreManager.Import(score).GetResultSafely().Value);
+                    importedScores.Add(scoreManager.Import(score).Value);
                 }
             });