- Removed no longer used StringBuiltinAssembler methods (DispatchOnStringEncodings, PointerToStringDataAtIndex)
- Removed no longer used Runtime functions (StringIncludes, StringIndexOf, StringIndexOfUnchecked).
- Overall builtin code size is reduced (652 bytes on Mac x64.release build), builtin size breakdown:

BEFORE
======
TFS Builtin, StringIndexOf, 1092
TFJ Builtin, StringPrototypeIncludes, 1784
TFJ Builtin, StringPrototypeIndexOf, 1536
Total = 4412

AFTER
=====
TFC Builtin, StringIndexOf, 2036 (+944)
TFJ Builtin, StringPrototypeIncludes, 1072 (-712)
TFJ Builtin, StringPrototypeIndexOf, 652 (-884)
Total = 3760 (-652)


Bug: v8:8996
Change-Id: I9a88c095e2097f7d570e58e744d6692dc524ddf4
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2660995
Commit-Queue: Peter Wong <peter.wm.wong@gmail.com>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72456}