remove unused scratchBuffer field

Author: Poker-sang

src/ImageSharp/Formats/Webp/BitWriter/BitWriterBase.cs

@@ -22,11 +22,6 @@ internal abstract class BitWriterBase
     /// </summary>
     private byte[] buffer;
 
-    /// <summary>
-    /// A scratch buffer to reduce allocations.
-    /// </summary>
-    private ScratchBuffer scratchBuffer;  // mutable struct, don't make readonly
-
     /// <summary>
     /// Initializes a new instance of the <see cref="BitWriterBase"/> class.
     /// </summary>