Remove redundant access check for Issues#destroy

We were already calling `authorize_admin_issue!` in a before filter with
the same permission checks, so this deleted check wasn't actually doing
anything.

app/controllers/issues_controller.rb

@@ -87,8 +87,6 @@ class IssuesController < ApplicationController
   end
 
   def destroy
-    return access_denied! unless can?(current_user, :admin_issue, @issue)
-
     @issue.destroy
 
     respond_to do |format|