Bug 1918681 - part 1: Allow moving the list-all-tabs button again r=dao

In bug 878551, the ability to move the list-all-tabs button was	removed
for a few reasons:

> Moving this widget to the navbar gives it a tiny scaled version of Toolbar.png (like a horizontal line).

> Moving it back to the tab strip make it misbehave. It disappears, but sometimes flickers into view when opening/closing a tab.

These issues are no longer present, so we should be able to allow moving
the list-all-tabs button once again.

Differential Revision: https://phabricator.services.mozilla.com/D224623

browser/base/content/navigator-toolbox.inc.xhtml

@@ -90,7 +90,7 @@
                        delegatesanchor="true"
                        badged="true"
                        data-l10n-id="tabs-toolbar-list-all-tabs"
-                       removable="false"/>
+                       removable="true"/>
       </hbox>
     </hbox>