drop redundant index_submissions_on_user_id

Change-Id: I6240393bf4d972075f3bbacc08bcadb4dd899b0f
Reviewed-on: https://gerrit.instructure.com/20793
Reviewed-by: Cody Cutrer <cody@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Brian Palmer <brianp@instructure.com>
QA-Review: Brian Palmer <brianp@instructure.com>

db/migrate/20130521161315_drop_redundant_submissions_index.rb

@@ -0,0 +1,12 @@
+class DropRedundantSubmissionsIndex < ActiveRecord::Migration
+  tag :predeploy
+
+  def self.up
+    # there's also a (unique) index on [:user_id, :assignment_id]
+    remove_index :submissions, [:user_id]
+  end
+
+  def self.down
+    add_index :submissions, [:user_id]
+  end
+end