Don't auto-install python-mode anymore

It's part of Emacs 24.3 now

core/prelude-packages.el

@@ -87,7 +87,6 @@
     ("\\.markdown\\'" markdown-mode markdown-mode)
     ("\\.md\\'" markdown-mode markdown-mode)
     ("\\.php\\'" php-mode php-mode)
-    ("\\.py\\'" python python-mode)
     ("\\.sass\\'" sass-mode sass-mode)
     ("\\.scala\\'" scala-mode2 scala-mode)
     ("\\.scss\\'" scss-mode scss-mode)