Delete always-zero ByteBufferWriter::start_.

Likely a left-over since cl https://codereview.webrtc.org/1821083002

BUG=None

Review-Url: https://codereview.webrtc.org/2440083002
Cr-Commit-Position: refs/heads/master@{#14732}
diff --git a/webrtc/base/bytebuffer.h b/webrtc/base/bytebuffer.h
index cd7b2c6..546c447 100644
--- a/webrtc/base/bytebuffer.h
+++ b/webrtc/base/bytebuffer.h
@@ -46,9 +46,9 @@
 
   ~ByteBufferWriter();
 
-  const char* Data() const { return bytes_ + start_; }
-  size_t Length() const { return end_ - start_; }
-  size_t Capacity() const { return size_ - start_; }
+  const char* Data() const { return bytes_; }
+  size_t Length() const { return end_; }
+  size_t Capacity() const { return size_; }
 
   // Write value to the buffer. Resizes the buffer when it is
   // neccessary.
@@ -77,7 +77,6 @@
 
   char* bytes_;
   size_t size_;
-  size_t start_;
   size_t end_;
 
   // There are sensible ways to define these, but they aren't needed in our code