Remove old validation message

Author: jamiebenstead

config/locales/en.yml

@@ -18,7 +18,6 @@ en:
       reference: "must be 5-6 digits (e.g., 100000)"
       reference_urn_exists: "URN number already exists"
       district_nces_id: "must be 7 digits (e.g., 0100000)"
-      district_nces_id_exists: "NCES ID already exists"
       school_roll_number: "must be numbers followed by letters (e.g., 01572D)"
       school_roll_number_exists: "School roll number already exists"
     invitation: