[Sync] Remove DropAttachments from AttachmentService and SyncData

sync/api/sync_data.h

Index: sync/api/sync_data.h
diff --git a/sync/api/sync_data.h b/sync/api/sync_data.h
index 4e5acd3da29629a72948621285022ba65ca24c25..6815896af3649ab7081271ecc169da8168e54bd7 100644
--- a/sync/api/sync_data.h
+++ b/sync/api/sync_data.h
@@ -190,15 +190,6 @@ class SYNC_EXPORT SyncDataRemote : public SyncData {
   void GetOrDownloadAttachments(
       const AttachmentIdList& attachment_ids,
       const AttachmentService::GetOrDownloadCallback& callback);
-
-  // Drop (delete from local storage) the attachments associated with this
-  // SyncData specified in |attachment_ids|. This method will not delete
-  // attachments from the server.
-  //
-  // |callback| will be invoked when the operation is complete (successfully
-  // or otherwise).
-  void DropAttachments(const AttachmentIdList& attachment_ids,
-                       const AttachmentService::DropCallback& callback);
 };
 
 // gmock printer helper.