Revert of Serializer: clear string hash for code serializer. (patchset #1 id:1 of https://codereview.chromium.org/1183483006/)

Reason for revert:
This led to the roll being stuck: https://codereview.chromium.org/1189863003/

Original issue's description:
> Serializer: clear string hash for code serializer.
>
> R=jochen@chromium.org
> BUG=v8:4179
> LOG=N
>
> Committed: https://crrev.com/a0342678e865337215f9195f983c6013b60e7641
> Cr-Commit-Position: refs/heads/master@{#29016}

TBR=jochen@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=v8:4179

Review URL: https://codereview.chromium.org/1186173004

Cr-Commit-Position: refs/heads/master@{#29065}