Remove elpy-interactive-python-command

The variable was there solely to allow users to easily enable
ipython from customize, but with 24.4, that's easy enough
already. Also, this had some bad interaction when users tried
to customize the python.el-specific interpreter variable in
addition to this one. So, removed.

Fixes #531

elpy.el

@@ -57,32 +57,6 @@
   :prefix "elpy-"
   :group 'languages)
 
-(defcustom elpy-interactive-python-command "python"
-  "Command to use for the interactive shell.
-
-Customize this option to use a different interactive shell. If
-the value starts with \"ipython\", it will set up python.el so
-that it deals with ipytohon's particular prompt and features.
-
-From your .emacs, you can use `elpy-use-ipython' and
-`elpy-use-cpython' instead."
-  :type '(choice (const :tag "Standard Python (python)" "python")
-                 (const :tag "Standard Python 2 (python2)" "python2")
-                 (const :tag "Standard Python 3 (python3)" "python3")
-                 (const :tag "IPython" "ipython")
-                 (string :tag "Other"))
-  :set (lambda (var val)
-         (set-default var val)
-         (if (string-match "ipython" val)
-             (elpy-use-ipython val)
-           (elpy-use-cpython val)))
-  ;; Don't use the default function because the default values are
-  ;; correct, and `elpy-use-cpython' is not available yet.
-  :initialize #'set-default
-  :safe (lambda (val)
-          (member val '("python" "python2" "python3" "ipython")))
-  :group 'elpy)
-
 (defcustom elpy-mode-hook nil
   "Hook run when `elpy-mode' is enabled.