Remove LegacyID from Ruleset

osu.Game/Rulesets/ILegacyRuleset.cs

@@ -0,0 +1,13 @@
+// Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
+// See the LICENCE file in the repository root for full licence text.
+
+namespace osu.Game.Rulesets
+{
+    public interface ILegacyRuleset
+    {
+        /// <summary>
+        /// Identifies the server-side ID of a legacy ruleset.
+        /// </summary>
+        int LegacyID { get; }
+    }
+}