Remove some non-standard code that doesn't do anything anyways.

While FireFox uses this to set the default value for the multiline flag,
it is nonstandard and slated for removal. The matching behaviour has
never been implemented in either JSC or V8, so there is little
web-compat risk.

The only possible risk could be someone depending on the ToBoolean()
behaviour of the flag, but this seems unlikely.

BUG=v8:3870
LOG=N
R=adamk@chromium.org, littledan@chromium.org, yangguo@chromium.org
CQ_INCLUDE_TRYBOTS=tryserver.chromium.linux:linux_chromium_rel_ng;tryserver.blink:linux_blink_rel

Review URL: https://codereview.chromium.org/1417733012

Cr-Commit-Position: refs/heads/master@{#31882}