web: Remove NoRepositoriesEnabledAlert and noRepositoriesEnabled flag (…

…sourcegraph#4128)

This fixes sourcegraph#4127 by

1. removing the `NoRepositoriesEnabledAlert`
2. deprecating the `noRepositoriesEnabled` `SiteFlag` and always have it
   be `false`
3. removing every usage of the `noRepositoriesEnabled` `SiteFlag` in the
   web app