Remove the self-healing for invalid parameters in the
CodeStubAssembler::SubString helper and the %SubString runtime function,
which is used as a fallback for the CodeStubAssembler implementation.
All call sites must do appropriate parameter validation anyways now that
the self-hosted JavaScript builtins using these helpers are gone, and we
have proper contracts with the uses.

Also remove the context parameter from the CodeStubAssembler::SubString
method, which is unnecessary, since this can no longer throw an
exception.

Bug: v8:5269, v8:6936, v8:7109, v8:7137
Change-Id: I19d93bad5f41faa0561c4561a48f78fcba99a549
Reviewed-on: https://chromium-review.googlesource.com/795720Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49702}