reverted split at Decoder, moved logic down

I'm done experimenting, sorry
- `Decoder` only returns a "Beatmap"`Decoder` now
- "Storyboard"`Decoder` is retrieved from a "Beatmap"`Decoder`
- moved parse methods down in the hierarchy where I forgot to do that
- renamed `OsuLegacyDecoderTest` to `LegacyDecoderTest`

osu.Game.Tests/Beatmaps/Formats/LegacyDecoderTest.cs

@@ -14,7 +14,7 @@ using osu.Game.Rulesets.Objects.Types;
 namespace osu.Game.Tests.Beatmaps.Formats
 {
     [TestFixture]
-    public class OsuLegacyDecoderTest
+    public class LegacyDecoderTest
     {
         [Test]
         public void TestDecodeMetadata()

osu.Game.Tests/Beatmaps/IO/OszArchiveReaderTest.cs

@@ -50,7 +50,7 @@ namespace osu.Game.Tests.Beatmaps.IO
 
                 BeatmapMetadata meta;
                 using (var stream = new StreamReader(reader.GetStream("Soleily - Renatus (Deif) [Platter].osu")))
-                    meta = Decoder.GetBeatmapDecoder(stream).DecodeBeatmap(stream).Metadata;
+                    meta = Decoder.GetDecoder(stream).DecodeBeatmap(stream).Metadata;
 
                 Assert.AreEqual(241526, meta.OnlineBeatmapSetID);
                 Assert.AreEqual("Soleily", meta.Artist);

osu.Game.Tests/osu.Game.Tests.csproj

@@ -89,7 +89,7 @@
     <Compile Include="Beatmaps\IO\OszArchiveReaderTest.cs" />
     <Compile Include="Beatmaps\IO\ImportBeatmapTest.cs" />
     <Compile Include="Resources\Resource.cs" />
-    <Compile Include="Beatmaps\Formats\OsuLegacyDecoderTest.cs" />
+    <Compile Include="Beatmaps\Formats\LegacyDecoderTest.cs" />
     <Compile Include="Visual\TestCaseBeatmapDetailArea.cs" />
     <Compile Include="Visual\TestCaseBeatmapDetails.cs" />
     <Compile Include="Visual\TestCaseBeatmapOptionsOverlay.cs" />