Remove themes.default from nl and pt-BR locales.

This translation has been supplanted by the flavours-related
translations.

config/locales/nl.yml

@@ -678,8 +678,6 @@ nl:
 
       <p>Originally adapted from the <a href="https://github.com/discourse/discourse">Discourse privacy policy</a>.</p>
     title: "%{instance} Terms of Service and Privacy Policy"
-  themes:
-    default: Mastodon
   time:
     formats:
       default: "%d %B %Y om %H:%M"

config/locales/pt-BR.yml

@@ -675,8 +675,6 @@ pt-BR:
 
       <p>Originalmente adaptado da <a href="https://github.com/discourse/discourse">política de privacidade do Discourse</a>.</p>
     title: "%{instance} Termos de Serviço e Política de Privacidade"
-  themes:
-    default: Mastodon
   time:
     formats:
       default: "%b %d, %Y, %H:%M"