Remove robots.txt

Google’s documentation says:

> robots.txt is not a mechanism for keeping a web page out of Google. To
> keep a web page out of Google, you should use noindex directives

We’ve implemented a noindex directive now, so we don’t need to serve
robots.txt any more.

app/navigation.py

@@ -256,7 +256,6 @@ class HeaderNavigation(Navigation):
         'returned_letters',
         'returned_letters_report',
         'revoke_api_key',
-        'robots',
         'send_messages',
         'send_notification',
         'send_one_off',
@@ -608,7 +607,6 @@ class MainNavigation(Navigation):
         'returned_letters_report',
         'revalidate_email_sent',
         'roadmap',
-        'robots',
         'security',
         'send_notification',
         'send_from_contact_list',
@@ -870,7 +868,6 @@ class CaseworkNavigation(Navigation):
         'revalidate_email_sent',
         'revoke_api_key',
         'roadmap',
-        'robots',
         'security',
         'send_messages',
         'send_notification',
@@ -1163,7 +1160,6 @@ class OrgNavigation(Navigation):
         'revalidate_email_sent',
         'revoke_api_key',
         'roadmap',
-        'robots',
         'security',
         'send_messages',
         'send_notification',