Removing the feature flags for new header and footer.

This is the clean-up task for removing the ENABLE_NEW_EDX_HEADER and ENABLE_NEW_EDX_FOOTER feature flags.
https://openedx.atlassian.net/browse/ECOM-136

Now the new header will be the default, and the new footer is still behind another feature flag "IS_EDX_DOMAIN".

@wedaly @dianakhuang @rlucioni One or two quick reviews, and I imagine I'll want to merge this only once we've flipped the feature flags in prod (you know, such as today / tomorrow), or should I go ahead anyway?

@sarina for context regarding headers and footers in Open edX. This means the new header will be showing up for open source. This header will mainly appear on registration / login / dashboard. It has slightly different styles.