Remove --harmony implications for incomplete features

This is in preparation for making --harmony and --es-staging synonyms.
The only remaining difference currently is block-scoping, which is still
implied by --harmony, to avoid regressing on a long-available feature.

Also removes the special-casing of --harmony-proxies.

R=adamk@chromium.org
BUG=

Review URL: https://codereview.chromium.org/693153004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@25115 ce2b1a6d-e550-0410-aec6-3dcde31c8c00