Revert of Turn on icu_case_mapping by default (patchset #3 id:40001 of https://codereview.chromium.org/2588963002/ )

Reason for revert:
Causes gc stress failures:
https://build.chromium.org/p/client.v8/builders/V8%20Linux64%20GC%20Stress%20-%20custom%20snapshot/builds/9920

Original issue's description:
> Turn on icu_case_mapping by default
>
> BUG=v8:4477, v8:4476
> TEST=test262/{built-ins,intl402}/Strings/*, webkit/fast/js/*,
>      mjsunit/string-case, intl/general/case*
>
> Review-Url: https://codereview.chromium.org/2588963002
> Cr-Commit-Position: refs/heads/master@{#41834}
> Committed: https://chromium.googlesource.com/v8/v8/+/7c79e23c34ea971947eedc6e42d8a882617c0e47

TBR=littledan@chromium.org,yangguo@chromium.org,jshin@chromium.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=v8:4477, v8:4476

Review-Url: https://codereview.chromium.org/2593543002
Cr-Commit-Position: refs/heads/master@{#41835}