session controller: remove dead session[:previous_uri] cookie.

session[:previous_uri] is never set so it's always nil.

app/controllers/sessions_controller.rb

@@ -8,7 +8,7 @@ class SessionsController < ApplicationController
 
     if session_creator.authenticate
       url = params[:url] if params[:url] && params[:url].start_with?("/")
-      redirect_to(url || session[:previous_uri] || posts_path, :notice => "You are now logged in")
+      redirect_to(url || posts_path, :notice => "You are now logged in")
     else
       redirect_to(new_session_path, :notice => "Password was incorrect")
     end