fixup! Bug 18905: Hide unwanted items from help menu

Bug 42740: Stop trying to hide "Restore previous session".

Firefox already handles this item. If there is no previous session to
restore (such as when using permanent private browsing) it is already
hidden.

browser/base/content/appmenu-viewcache.inc.xhtml

@@ -172,8 +172,7 @@
       <toolbarbutton id="appMenu-restoreSession"
                      data-l10n-id="appmenu-restore-session"
                      class="subviewbutton"
-                     command="Browser:RestoreLastSession"
-                     hidden="true"/>
+                     command="Browser:RestoreLastSession"/>
       <toolbarseparator/>
       <toolbarbutton id="appMenuClearRecentHistory"
                      data-l10n-id="appmenu-clear-history"