Remove release notes from event payload

The release notes are provided by Squirrel on Mac but not by Squirrel for
Windows and the release notes package pulls them down manually anyway
so this field is no longer needed.

Checking for the presence of the release notes previously was preventing the
event from firing on Windows which would cause old release notes to show in
the package.

Closes #3757

spec/atom-spec.coffee

@@ -47,9 +47,8 @@ describe "the `atom` global", ->
         updateAvailableHandler.callCount > 0
 
       runs ->
-        {releaseVersion, releaseNotes} = updateAvailableHandler.mostRecentCall.args[0]
+        {releaseVersion} = updateAvailableHandler.mostRecentCall.args[0]
         expect(releaseVersion).toBe 'version'
-        expect(releaseNotes).toBe 'notes'
 
   describe "loading default config", ->
     it 'loads the default core config', ->

src/browser/auto-update-manager.coffee

@@ -47,7 +47,7 @@ class AutoUpdateManager
       @setState(ErrorState)
       console.error "Error Downloading Update: #{message}"
 
-    autoUpdater.on 'update-downloaded', (event, @releaseNotes, @releaseVersion) =>
+    autoUpdater.on 'update-downloaded', (event, releaseNotes, @releaseVersion) =>
       @setState(UpdateAvailableState)
       @emitUpdateAvailableEvent(@getWindows()...)
 
@@ -61,9 +61,9 @@ class AutoUpdateManager
         @setState(UnsupportedState)
 
   emitUpdateAvailableEvent: (windows...) ->
-    return unless @releaseVersion? and @releaseNotes
+    return unless @releaseVersion?
     for atomWindow in windows
-      atomWindow.sendMessage('update-available', {@releaseVersion, @releaseNotes})
+      atomWindow.sendMessage('update-available', {@releaseVersion})
 
   setState: (state) ->
     return if @state is state

src/window-event-handler.coffee

@@ -31,8 +31,8 @@ class WindowEventHandler
           atom.updateAvailable(detail)
 
           # FIXME: Remove this when deprecations are removed
-          {releaseVersion, releaseNotes} = detail
-          detail = [releaseVersion, releaseNotes]
+          {releaseVersion} = detail
+          detail = [releaseVersion]
           if workspaceElement = atom.views.getView(atom.workspace)
             atom.commands.dispatch workspaceElement, "window:update-available", detail