Remove code for full-width feature.

The full-width feature has been turned off for
about four years.

static/js/feature_flags.js

@@ -4,9 +4,6 @@ var exports = {};
 
 exports.load_server_counts = false;
 
-// Experimental modification to support much wider message views.
-exports.full_width = false;
-
 // The features below have all settled into their final states and can
 // be removed when we get a chance
 exports.mark_read_at_bottom = true;