Revert of [crankshaft] Re-add fast-case for string add left/right.

Revert of [crankshaft] Re-add fast-case for string add left/right. (patchset #1 id:1 of https://codereview.chromium.org/1339053002/ )

Reason for revert:
[Sheriff] Fails mozilla with deadcode:
http://build.chromium.org/p/client.v8/builders/V8%20Linux%20-%20deadcode/builds/5357

Original issue's description:
> [crankshaft] Re-add fast-case for string add left/right.
>
> Now the StringAddStub can optionally convert it's parameters to strings
> (following the rules for the addition operator). This could be further
> optimized once we have a ToPrimitiveStub, but it should be sufficient
> for the moment.
>
> Also removed the unused Strength parameter to the HStringAdd operator,
> because string addition does not depend on language mode.
>
> CQ_INCLUDE_TRYBOTS=tryserver.v8:v8_win_nosnap_shared_rel,v8_linux_nosnap_dbg
> R=mstarzinger@chromium.org
> BUG=v8:4307
> LOG=n
>
> Committed: https://crrev.com/d261849e53fbf8c36efae42d478271f87acff70f
> Cr-Commit-Position: refs/heads/master@{#30726}

TBR=mstarzinger@chromium.org,jarin@chromium.org,bmeurer@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=v8:4307
Committed: https://crrev.com/c5a4c39873b2dcb0c79f3f6542a4961f70f7be61
Cr-Commit-Position: refs/heads/master@{#30727}

[Michael Achenbach, 2015-09-15 06:40:20]

Created Revert of [crankshaft] Re-add fast-case for string add left/right.

[commit-bot: I haz the power, 2015-09-15 06:40:32]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1347473003/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1347473003/1

[commit-bot: I haz the power, 2015-09-15 06:40:42]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-09-15 06:41:03]

Patchset 1 (id:??) landed as
https://crrev.com/c5a4c39873b2dcb0c79f3f6542a4961f70f7be61
Cr-Commit-Position: refs/heads/master@{#30727}