Bug 1491243 - remove subscribe button, menu items and subscription section out of feed previews, r=florian

This removes subscribe UI and functionality from the main browser window,
the page info window, and from feed previews. It may leave some stray strings
in subscribe.properties/dtd, which will be removed in bug 1477669 when the
preview code goes away completely.

Differential Revision: https://phabricator.services.mozilla.com/D5982

browser/base/content/browser.js

@@ -3661,7 +3661,6 @@ var newWindowButtonObserver = {
 const DOMEventHandler = {
   init() {
     let mm = window.messageManager;
-    mm.addMessageListener("Link:AddFeed", this);
     mm.addMessageListener("Link:LoadingIcon", this);
     mm.addMessageListener("Link:SetIcon", this);
     mm.addMessageListener("Link:SetFailedIcon", this);
@@ -3671,11 +3670,6 @@ const DOMEventHandler = {
 
   receiveMessage(aMsg) {
     switch (aMsg.name) {
-      case "Link:AddFeed":
-        let link = {type: aMsg.data.type, href: aMsg.data.href, title: aMsg.data.title};
-        FeedHandler.addFeed(link, aMsg.target);
-        break;
-
       case "Link:LoadingIcon":
         if (aMsg.data.canUseForTab) {
           this.setPendingIcon(aMsg.target);
@@ -4708,9 +4702,6 @@ var XULBrowserWindow = {
         aStateFlags & nsIWebProgressListener.STATE_IS_NETWORK) {
 
       if (aRequest && aWebProgress.isTopLevel) {
-        // clear out feed data
-        browser.feeds = null;
-
         // clear out search-engine data
         browser.engines = null;
       }
@@ -4940,7 +4931,6 @@ var XULBrowserWindow = {
   },
 
   asyncUpdateUI() {
-    FeedHandler.updateFeeds();
     BrowserSearch.updateOpenSearchBadge();
   },