Refactor to add separate letter branding client

We were getting all letter logos from a method in the email branding
client. Since we will be adding more client methods to deal with
letters, it makes things clearer to separate the email and letter
branding clients.

app/notify_client/email_branding_client.py

@@ -23,9 +23,6 @@ class EmailBrandingClient(NotifyAdminAPIClient):
                 return branding['id']
         return None
 
-    def get_letter_email_branding(self):
-        return self.get(url='/dvla_organisations')
-
     @cache.delete('email_branding')
     def create_email_branding(self, logo, name, text, colour, domain, brand_type):
         data = {