Since the removal of Crankshaft there's no use for StringAdd with
pretenuring anymore, so we can remove the extra code and builtins.

Bug: v8:8015
Change-Id: If178c6f1d08841428f42b1baece231268cdae2ad
Reviewed-on: https://chromium-review.googlesource.com/1213206
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55843}