Removed unused let blocks (#7446)

The bodies of these blocks were never evaluated.

spec/requests/export_csv_controller_spec.rb

@@ -1,8 +1,6 @@
 require "rails_helper"
 
 describe ExportCsvController do
-  let(:export_filename) { "user-archive-codinghorror-150115-234817-999.csv.gz" }
-
   context "while logged in as normal user" do
     let(:user) { Fabricate(:user) }
     before { sign_in(user) }