Disable jQuery Migrate

A simple plugin to disable loading of jquery-migrate on the frontend due to a XSS vulerability of jQuery Migrate 1.2.1. More details here: https://github.com/jquery/jquery-migrate/issues/36

It's also one thing less to load where it counts.

Props to people in this thread: https://wordpress.org/support/topic/removing-the-default-jquery-and-migrate-from-the-header-in-last-version-of-wp?replies=6.