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/components/customizableui/CustomizableUI.sys.mjs

@@ -4194,7 +4194,6 @@ var CustomizableUIInternal = {
 
     // Normally these aren't removable, but for this operation only we need to move them
     changeWidgetRemovability("tabbrowser-tabs", true);
-    changeWidgetRemovability("alltabs-button", true);
 
     if (toVertical) {
       lazy.log.debug(
@@ -4249,7 +4248,6 @@ var CustomizableUIInternal = {
     );
     this.setToolbarVisibility(CustomizableUI.AREA_TABSTRIP, !toVertical);
     changeWidgetRemovability("tabbrowser-tabs", false);
-    changeWidgetRemovability("alltabs-button", false);
 
     for (let [win] of gBuildWindows) {
       win.TabBarVisibility.update(true);