Simplify 'ReadOnlySequenceStream.Dispose()'

Author: Sergio0694

src/CommunityToolkit.HighPerformance/Streams/ReadOnlySequenceStream.cs

@@ -298,11 +298,6 @@ public sealed override void WriteByte(byte value)
     /// <inheritdoc/>
     protected override void Dispose(bool disposing)
     {
-        if (this.disposed)
-        {
-            return;
-        }
-
         this.disposed = true;
     }
 }