Delete StringCharCodeAtGenerator, as it's no longer used.

Bug: 
Change-Id: I05294fac69671c0652742d120de9ea17e9031518
Reviewed-on: https://chromium-review.googlesource.com/613164Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Commit-Queue: Martyn Capewell <martyn.capewell@arm.com>
Cr-Commit-Position: refs/heads/master@{#47339}