Remove unused internationalization key

It looks like it isn't used since we replaced the order selector with
order links in debates a few years ago, in commit 7eda0b288.

config/i18n-tasks.yml

@@ -187,7 +187,6 @@ ignore_unused:
   - "polls.index.section_header.*"
   - "polls.index.orders.*"
   - "dashboard.recommended_actions.*"
-  - "debates.index.select_order"
   - "debates.index.orders.*"
   - "debates.index.section_header.*"
   - "proposals.index.orders.*"

config/locales/en/general.yml

@@ -113,7 +113,6 @@ en:
       search_results:
         one: " containing the term <strong>'%{search_term}'</strong>"
         other: " containing the term <strong>'%{search_term}'</strong>"
-      select_order: Order by
       start_debate: Start a debate
       title: Debates
       section_header:

config/locales/es/general.yml

@@ -113,7 +113,6 @@ es:
       search_results:
         one: " que contiene <strong>'%{search_term}'</strong>"
         other: " que contienen <strong>'%{search_term}'</strong>"
-      select_order: Ordenar por
       start_debate: Empieza un debate
       title: Debates
       section_header: