Remove IsAlive from SampleChannel to let childern define their own

This commit also adds one for SampleChannelVirtual.
It is needed to let childern use IsAlive from parent of SampleChannel.

Author: hwsmm

osu.Framework/Audio/Sample/SampleChannel.cs

@@ -44,8 +44,6 @@ protected override void UpdateState()
 
         public virtual bool Looping { get; set; }
 
-        public override bool IsAlive => base.IsAlive && Playing;
-
         public virtual ChannelAmplitudes CurrentAmplitudes { get; } = ChannelAmplitudes.Empty;
 
         #region Mixing

osu.Framework/Audio/Sample/SampleChannelVirtual.cs

@@ -13,6 +13,8 @@ internal class SampleChannelVirtual : SampleChannel
 
         public override bool Playing => playing;
 
+        public override bool IsAlive => base.IsAlive && Playing;
+
         public SampleChannelVirtual(string name)
             : base(name)
         {