cleanup: Remove default Chrome extension ID

It makes for a bad first-time experience for users, since the desktop sharing
button will be visible, but it will never work.

Also get rid of the now deprecated `desktopSharingChromeMethod` option.

config.js

     //focusUserJid: 'focus@auth.jitsi-meet.example.com', // The real JID of focus participant - can be overridden here
     //defaultSipNumber: '', // Default SIP number
 
-    // Desktop sharing method. Can be set to 'ext', 'webrtc' or false to disable.
-    desktopSharingChromeMethod: 'ext',
     // The ID of the jidesha extension for Chrome.
-    desktopSharingChromeExtId: 'diibjkoicjeejcmhdnailmkgecihlobk',
+    desktopSharingChromeExtId: null,
+    // Whether desktop sharing should be disabled on Chrome.
+    desktopSharingChromeDisabled: true,
     // The media sources to use when using screen sharing with the Chrome
     // extension.
     desktopSharingChromeSources: ['screen', 'window', 'tab'],

doc/manual-install.md

     },
     useNicks: false,
     bosh: '//jitsi.example.com/http-bind', // FIXME: use xep-0156 for that
-    desktopSharing: 'false' // Desktop sharing method. Can be set to 'ext', 'webrtc' or false to disable.
     //chromeExtensionId: 'diibjkoicjeejcmhdnailmkgecihlobk', // Id of desktop streamer Chrome extension
     //minChromeExtVersion: '0.1' // Required version of Chrome extension
 };