[cleanup] Remove dead Vector::set_start method

R=mstarzinger@chromium.org

Bug: v8:7754
Change-Id: I67ef360bcc4d49007e29036b49c87b6549e8d8e2
Reviewed-on: https://chromium-review.googlesource.com/1116778Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54049}

src/vector.h

@@ -150,9 +150,6 @@ class Vector {
     return true;
   }
 
- protected:
-  void set_start(T* start) { start_ = start; }
-
  private:
   T* start_;
   size_t length_;