This patch removes the StringConstructor_ConstructStub builtin,
merging its functionality into the refactored StringConstructor
TurboFan builtin.

This brings us closer to our goal of deprecating the `construct_stub`
field in `SharedFunctionInfo`.

Bug: v8:7503
Change-Id: Ie98520c652f49dda91eff2fc51263611f29e0ebe
Reviewed-on: https://chromium-review.googlesource.com/942882
Commit-Queue: Mathias Bynens <mathias@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51660}