Check for valid segments before returning recipients

We were getting a warning by CodeQL regarding a possible code injection
in the `send(segment)` code.

In practice, this wasn't a big deal because the `self.recipients` method
is only called in the admin section, meaning only admin users could try
to take advantage of the code injection, and because this code is rarely
called with an invalid segment due to several conditions in the code
checking that the user segment is valid, with the only exception being
the `generate_csv` action in the `Admin::EmailsDownloadController`.

In any case, now we're checking that the segment is valid before calling
the `send` method. Since now we're making sure that the segment is valid
in the `recipients` method itself, we can remove this check from methods
calling it.

app/models/admin_notification.rb

@@ -13,7 +13,7 @@ class AdminNotification < ApplicationRecord
   before_validation :complete_link_url
 
   def list_of_recipients
-    UserSegments.recipients(segment_recipient) if valid_segment_recipient?
+    UserSegments.recipients(segment_recipient)
   end
 
   def valid_segment_recipient?