Delete duplicated extending layout plugin pages

Summary:
This content is currently duplicated in the fb internal section. So deleting it.
It's also in public here: https://www.internalfb.com/intern/staticdocs/flipper/docs/extending/layout-inspector

The only content in these files that isn't an exact dupe, is the iOS "Adding the plugin to a new app". But that's already covered in the plugin setup page: https://www.internalfb.com/intern/staticdocs/flipper/docs/setup/layout-plugin

Reviewed By: passy

Differential Revision: D25563321

fbshipit-source-id: 26d3b8c2c01c170cd440ae78bfaff20a13f90d37

website/sidebars.js

@@ -103,12 +103,6 @@ module.exports = {
       {
         'Internal Plugins': [
           'fb/plugins',
-          {
-            Layout: [
-              'fb/layout-extending-android-layout-inspector',
-              'fb/layout-extending-ios-layout-inspector',
-            ],
-          },
           'fb/Memory-Tools',
           'fb/Navigation-Plugin',
           'fb/supporting-feed-inspector',