Remove Isolated Worlds from Sky

These were a feature for allowing multiple
scripting contexts to access the same global
state without leaking wrappers between them.
For example, if the inspector wanted to
modify window.Array.dangerousFunction = ...
it wouldn't want the author's content to have
access to that.

This feature is not part of Sky's security model
and thus this is just dead code.

I tried to remove worlds all together, but there
is something special about how we use a
"fake" world (which is neither main nor isolated)
for GC, regexp and testing.

R=rafaelw@chromium.org, abarth@chromium.org
BUG=

Review URL: https://codereview.chromium.org/776143003