Delegate file deletion to ImportTask to allow overriding it

osu.Game/Database/ImportTask.cs

@@ -51,6 +51,15 @@ namespace osu.Game.Database
                 : getReaderFrom(Path);
         }
 
+        /// <summary>
+        /// Deletes the file that is encapsulated by this <see cref="ImportTask"/>.
+        /// </summary>
+        public virtual void DeleteFile()
+        {
+            if (File.Exists(Path))
+                File.Delete(Path);
+        }
+
         /// <summary>
         /// Creates an <see cref="ArchiveReader"/> from a stream.
         /// </summary>