Remove Internal.emptyData, not generated since apple#1028

Author: cburrows

Sources/SwiftProtobuf/Internal.swift

@@ -20,13 +20,6 @@ import Foundation
 /// implementations. NOT INTENDED TO BE CALLED BY CLIENTS.
 public enum Internal {
 
-  /// A singleton instance of an empty data that is used by the generated code
-  /// for default values. This is a performance enhancement to work around the
-  /// fact that the `Data` type in Swift involves a new heap allocation every
-  /// time an empty instance is initialized, instead of sharing a common empty
-  /// backing storage.
-  public static let emptyData = Data()
-
   /// Helper to loop over a list of Messages to see if they are all
   /// initialized (see Message.isInitialized for what that means).
   public static func areAllInitialized(_ listOfMessages: [Message]) -> Bool {