Drop #16488 patch

Our legacy/trac#16488 (moved) patch was intended to hide the Sign in to Sync button in the hamburger menu. Unfortuately, reviewers at Mozilla weren't sure it was a good idea, and then it stopped working correctly when I rebased to FF45ESR.

We introduced torbutton patch legacy/trac#18743 (moved) instead. So I believe we can revert (or remove) our legacy/trac#16488 (moved) patch from tor-browser.git.