Now that SimpleStringBuilder is only used in conversions.cc, it can be
moved there making it easier to assess its safety and limit further
use of this potentially unsafe API. (Additionally unused methods Reset
and size are removed).

Bug: v8:11917
Change-Id: I0515fe4f34bb8f7e7ea464b75394fa3d03939af1
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2978253
Commit-Queue: Dan Elphick <delphick@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#75300}