It hides the SF logo from Classic - it works with custom logos too
// ==UserScript==
// @name Hide SFDC classic logo
// @namespace window
// @version 0.1
// @description It hides the SF logo from Classic - it works with custom logos too
// @author You
// @match https://*.salesforce.com/**
// @grant none
// ==/UserScript==
(function() {
'use strict';
document.getElementById("phHeaderLogoImage").style.display = "none";
})();