[runtime] Remove obsolete %_StringAdd intrinsic.

[runtime] Remove obsolete %_StringAdd intrinsic.

The %_StringAdd intrinsic is not used anymore, so no need to keep the
code around.

R=yangguo@chromium.org

Committed: https://crrev.com/de9ebd82d011bcaaa9488cfdac6b5c2e5d68a2c4
Cr-Commit-Position: refs/heads/master@{#31822}

[Benedikt Meurer, 2015-11-05 11:58:49]

Hey Yang,

Dead code should be dead! Red-only!
Please take a look.

Thanks,
Benedikt

[Yang, 2015-11-05 12:00:26]

lgtm!

[Benedikt Meurer, 2015-11-05 12:00:33]

The CQ bit was checked by bmeurer@chromium.org

[commit-bot: I haz the power, 2015-11-05 12:00:44]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1420283019/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1420283019/1

[commit-bot: I haz the power, 2015-11-05 12:16:49]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-11-05 12:17:11]

Patchset 1 (id:??) landed as
https://crrev.com/de9ebd82d011bcaaa9488cfdac6b5c2e5d68a2c4
Cr-Commit-Position: refs/heads/master@{#31822}