Simple CSS rule takes care of it.
Browsers have no native capability to implement user CSS (except older Firefox and very old Chrome versions), so either Adblock Plus, uBlock Origin, Stylus, Stylish, reStyle for Firefox, or Stylus, Adblock, uBlock origin, or whatever else exists can be used.
Edge, Opera, and Safari have some options also. Pretty sure IE users are totally out of luck except for possible use of user stylesheets (see below the Stylish rule). Edge, and Safari users have adblocker options mostly, but Edge apparently can use TamperMonkey addon to implement userstyles (styles have to be converted to scripts to be used.)
(about 75 addons - many I have tweaked - are why I have stuck with Firefox since Phoenix 0.5, and why I stick with older Firefox versions).
The adblockers can implement the CSS via element hiding rules, and the ones newer than the very old versions I use (Firefox Adblock Plus v2.6.13 Nov 2015), have capability to right click on things and create the hiding rules for you. I create my rules manually by inspecting with Firebug and writing the rule myself as I would for Stylish if I were going to go that route. (I use Stylish v1.4.31 in Firefox 31)
Adblock Plus rule
community.ebay.com##navbar.top-menus.header-navigation-main>div.menu-wrap>div.left-side-menu
Stylish rule
@-moz-document domain('community.ebay.com'){
navbar.top-menus.header-navigation-main>div.menu-wrap>div.left-side-menu
{display:none !important;}
}
The Stylish rule can be pasted directly in a usercontent.css file in the Firefox profile to preclude having to install any addons. Chrome took away usercontent capability a while back, and Safari, Edge, IE, and Opera may (still?) allow user styles https://beamtic.com/custom-style-sheets
GreaseMonkey/TamperMonkey script
// ==UserScript==
// @name Hide eBay Forum NavMenu
// @namespace local
// @description Hide eBay Community Forum Dropdown Nav Menu
// @include https://community.ebay.com/*
// @version 0.1
// @grant none
// ==/UserScript==
var navmenu=document.querySelectorAll('navbar.top-menus.header-navigation-main>\
div.menu-wrap>div.left-side-menu');
if(navmenu[0] != null)navmenu.setAttribute('style', 'display:none', 'important');(The above is the type of thing that is amenable to adding to a general eBay Community forums fixups script as just another rule instead of having 49 separate scripts).
Adblock Plus element hiding rules seem to be (from what I've read and been told) portable to the other adblockers, and Stylish rules seem to also be portable to the other user content enabling addons (uBlock origin, Stylus, etc). Simple GreaseMonkey scripts port to TamperMonkey directly (I think)
Note that Stylish has run into troubles* recently - the addon changed ownership and went rogue dirtbag. Stylus appears to be a "clean" alternative for Firefox. Don't know what the alternatives are for Chrome.
*https://arstechnica.com/information-technology/2018/07/stylish-extension-with-2m-downloads-banished-...
(one can often divine lots of useful info from reading the comments on an Arstechnica article, like alternatives or workarounds)
https://www.ghacks.net/2017/01/04/major-stylish-add-on-changes-in-regards-to-privacy/
