NeverNewTab.user.js

// ==UserScript==
// @name        NeverNewTab
// @description Never open a new tab if you don't want it
// @namespace   http//roberg.nu/NeverNewTab
// @license WTFPL
// @include     http*
// @grant       none
// @version 0.0.1
// @encoding utf-8
//
// @run-at document-start
// ==/UserScript==

;(function nnt_init() {
  function fix() {
    var as = document.querySelectorAll('a');
    [].forEach.call(as, function (a) {
      if (a.target !== '') {
        a.target = '';
        console.debug('Anchor with href', a.href, 'wanted to open in a new tab.');
      }
    });

    setTimeout(fix, 1000);
  }

  try {
    var readyState = document.readyState;
    if (readyState === "complete" || readyState === "loaded") {
      fix();
    } else {
      document.addEventListener('DOMContentLoaded', fix);
    }
  } catch (e) {
    setTimeout(nnt_init, 100);
  }
}());