Enable ES6 Symbols by default

In doing so also remove all references to the --harmony-symbols flag.
Due to the way context snapshotting works, it's not possible to simply enable
the flag by default.

BUG=v8:2158
LOG=Y
R=dslomov@chromium.org

Review URL: https://codereview.chromium.org/421313004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22831 ce2b1a6d-e550-0410-aec6-3dcde31c8c00