Dashboard-left-panel-hide.user.js

// ==UserScript==
// @name        Dashboard-left-panel-hide
// @namespace   userscripts-mirror.org
// @description Hide left panel when multiple dashboards
// @include     http://*/secure/Dashboard*
// @require			https://ajax.googleapis.com/ajax/libs/jquery/3.3.1/jquery.min.js
// @version     2
// @grant       none
// ==/UserScript==

// works for Firefox, updated to work with JIRA v7.x
// adapted from
// https://answers.atlassian.com/questions/50924/dashboard-how-do-i-hide-the-dashboard-selector-on-the-left

var removeLeftPanel = function() {
  jQuery("div.dashboard-tabs.tabs.vertical").remove();
  jQuery("#dashboard").attr("class", "dashboard");
};


window.addEventListener("load", removeLeftPanel, true);