RemoveMessengerMobileAdBanner.user.js

// ==UserScript==
// @name         RemoveMessengerMobileAdBanner
// @namespace    http://tampermonkey.net/
// @version      0.1
// @description  Removes the annoying ad banner form the top of Messenger. 
// @author       Midiparse
// @match        https://www.messenger.com/*
// @grant        none
// ==/UserScript==
/* jshint -W097 */
'use strict';

/*!
 * contentloaded.js
 *
 * Author: Diego Perini (diego.perini at gmail.com)
 * Summary: cross-browser wrapper for DOMContentLoaded
 * Updated: 20101020
 * License: MIT
 * Version: 1.2
 *
 * URL:
 * http://javascript.nwbox.com/ContentLoaded/
 * http://javascript.nwbox.com/ContentLoaded/MIT-LICENSE
 *
 */

// @win window reference
// @fn function reference
function contentLoaded(win, fn) {

	var done = false, top = true,

	doc = win.document,
	root = doc.documentElement,
	modern = doc.addEventListener,

	add = modern ? 'addEventListener' : 'attachEvent',
	rem = modern ? 'removeEventListener' : 'detachEvent',
	pre = modern ? '' : 'on',

	init = function(e) {
		if (e.type == 'readystatechange' && doc.readyState != 'complete') return;
		(e.type == 'load' ? win : doc)[rem](pre + e.type, init, false);
		if (!done && (done = true)) fn.call(win, e.type || e);
	},

	poll = function() {
		try { root.doScroll('left'); } catch(e) { setTimeout(poll, 50); return; }
		init('poll');
	};

	if (doc.readyState == 'complete') fn.call(win, 'lazy');
	else {
		if (!modern && root.doScroll) {
			try { top = !win.frameElement; } catch(e) { }
			if (top) poll();
		}
		doc[add](pre + 'DOMContentLoaded', init, false);
		doc[add](pre + 'readystatechange', init, false);
		win[add](pre + 'load', init, false);
	}

}

contentLoaded(window, function(event) {
    var banner = document.getElementsByClassName('_s15');
    if(banner && banner.length && banner[0]) {
        banner[0].parentNode.removeChild(banner[0]);
    }
});