Remove the DateExtension

The DateExtension tried to kill unload handlers that delayed unload by
sleeping. However, if a page does this on purpose, it is easy to work
around this, plus running v8::Extensions slows down context creation.

BUG=299746
R=abarth@chromium.org

Review URL: https://codereview.chromium.org/25001004

git-svn-id: svn://svn.chromium.org/blink/trunk@160785 bbb929c8-8fbe-4397-9dbb-9b2b20218538