Almost right... Remove everything and paste this:
// ==UserScript==
// @name Hide BitcoinTalk Username and Avatar
// @include https://bitcointalk.org/*
// @version 1
// @grant none
// ==/UserScript==
try {
document.getElementById("smfheader").style.visibility = "hidden";
document.getElementById("hellomember").style.visibility = "hidden";
document.getElementsByClassName("avatar")[0].style.visibility = "hidden";
} catch { }
I also added a small line on it to only work on BitcoinTalk.
This removes the username, avatar, and the "Bitcoin Forum" text at the top. You can comment/remove the second line ("hellomember") if you don't want to hide your username.