Remove unused GetUnderlyingStream method

osu.Game.Tests/Scores/IO/ImportScoreTest.cs

@@ -236,8 +236,6 @@ public override void Dispose()
             }
 
             public override IEnumerable<string> Filenames => new[] { "test_file.osr" };
-
-            public override Stream GetUnderlyingStream() => new MemoryStream();
         }
     }
 }

osu.Game/IO/Archives/ArchiveReader.cs

@@ -45,7 +45,5 @@ public async Task<byte[]> GetAsync(string name)
                 return buffer;
             }
         }
-
-        public abstract Stream GetUnderlyingStream();
     }
 }

osu.Game/IO/Archives/LegacyDirectoryArchiveReader.cs

@@ -28,7 +28,5 @@ public override void Dispose()
         }
 
         public override IEnumerable<string> Filenames => Directory.GetFiles(path, "*", SearchOption.AllDirectories).Select(f => f.Replace(path, string.Empty).Trim(Path.DirectorySeparatorChar)).ToArray();
-
-        public override Stream GetUnderlyingStream() => null;
     }
 }

osu.Game/IO/Archives/LegacyFileArchiveReader.cs

@@ -28,7 +28,5 @@ public override void Dispose()
         }
 
         public override IEnumerable<string> Filenames => new[] { Name };
-
-        public override Stream GetUnderlyingStream() => null;
     }
 }

osu.Game/IO/Archives/ZipArchiveReader.cs

@@ -45,7 +45,5 @@ public override void Dispose()
         }
 
         public override IEnumerable<string> Filenames => archive.Entries.Select(e => e.Key).ExcludeSystemFileNames();
-
-        public override Stream GetUnderlyingStream() => archiveStream;
     }
 }