Bug 1886892 - Part 1: (ESR115) Remove non-SHIP code from desktop SessionStore, r=#sessionstore-reviewers, a=dmeehan


In bug 1804140, we shipped SHIP everywhere on desktop. Unfortunately, we
never removed the now-dead code from SessionStore.

This patch removes the easy to identify SessionStore code for non-SHIP,
now that it is a fully unsupported configuration.

Differential Revision: https://phabricator.services.mozilla.com/D209145