Remove unused KeyValue model (#3206).

db/migrate/20170709190409_drop_key_values.rb

@@ -0,0 +1,9 @@
+class DropKeyValues < ActiveRecord::Migration
+  def up
+    drop_table :key_values
+  end
+
+  def down
+    raise ActiveRecord::IrreversibleMigration, "Can't recover the lost data"
+  end
+end