Backed out 4 changesets (bug 1853698, bug 1853763, bug 1846498) for causing mochitest failures on browser_privatebrowsing_resetPBM.js & browser_1089591_still_customizable_after_reset CLOSED TREE

Backed out changeset 15945b5a02c7 (bug 1853698)
Backed out changeset b396820a7a46 (bug 1853763)
Backed out changeset 535f72d66f52 (bug 1846498)
Backed out changeset 439c5b41701a (bug 1846498)

browser/components/customizableui/CustomizableUI.sys.mjs

@@ -57,7 +57,7 @@ const kSubviewEvents = ["ViewShowing", "ViewHiding"];
  * The current version. We can use this to auto-add new default widgets as necessary.
  * (would be const but isn't because of testing purposes)
  */
-var kVersion = 20;
+var kVersion = 19;
 
 /**
  * Buttons removed from built-ins by version they were removed. kVersion must be
@@ -249,7 +249,6 @@ var CustomizableUIInternal = {
       "downloads-button",
       AppConstants.MOZ_DEV_EDITION ? "developer-button" : null,
       "fxa-toolbar-menu-button",
-      "reset-pbm-toolbar-button",
     ].filter(name => name);
 
     this.registerArea(
@@ -659,18 +658,6 @@ var CustomizableUIInternal = {
         ...addonsPlacements,
       ];
     }
-
-    // Add the PBM reset button as the right most button item
-    if (currentVersion < 20) {
-      let navbarPlacements = gSavedState.placements[CustomizableUI.AREA_NAVBAR];
-      // Place the button as the first item to the left of the hamburger menu
-      if (
-        navbarPlacements &&
-        !navbarPlacements.includes("reset-pbm-toolbar-button")
-      ) {
-        navbarPlacements.push("reset-pbm-toolbar-button");
-      }
-    }
   },
 
   _updateForNewProtonVersion() {