Found this while looking at MDL-42869.

It seems that since 2009 (commit 34a2777ccb973f2fc093249fcc3a8bb09c062b5d), Moodle inserts two meta tags to "allow control of cache without touching https headers". This technique seems a little outdated and the W3C validator report invalid "http-equiv" content and make the document invalid.
Since Moodle already handle cache with HTTP Headers, it seems we can remove this code.