Downgrade custom-theme-load-path into a defvar

[Artur Malabarba <bruce.connor.am@gmail.com>]

`custom-theme-load-path' is a defcustom, but setting it via the
customize interface is a trap for the user. Because installed themes
commonly add themselves to this variable, its value is not fit for
being saved (because it will permanently remember dirs that don't
exist anymore).
This is aggravated by the fact that Emacs always applies the `user'
theme on top of any theme that's loaded, since this will remove apply
the old's variable value and remove any new directories that had been
added by themes themselves.

For an instance of the problem in the wild, see
http://emacs.stackexchange.com/q/2112/50

I'd like to downgrade this defcustom into a defvar, so that users
cannot customize it by accident. It should not be a defcustom in the
same way that load-path isn't.

Would anyone object?


Best,
Artur