Revert `open-line' electric-indent sensitivity

* lisp/simple.el (open-line): Remove electric indent code.
  (electric-indent-just-newline): Don't declare.

* test/automated/simple-test.el (open-line-indent): Adjust test.

This partly reverts Artur Malabarba's change that added electric
indent sensitivity to `open-line' (Oct 24 22:26:27 2015 +0100, git
commit bd4f04f86), and adjusts a new test he added right afterwards
(Sat Oct 24 23:43:06 2015 +0100, git commit 207f235e3) accordingly.
However, the new INTERACTIVE argument to `open-line', which he also
added in the first commit, is not reverted here.

See the thread "Questioning the new behavior of `open-line'." on the
Emacs Devel mailing list, and in particular this message:

  From: Artur Malabarba
  Subject: Re: Questioning the new behavior of `open-line'.
  To: Karl Fogel
  Cc: David Kastrup, Pierpaolo Bernardi, emacs-devel
  Date: Wed, 18 Nov 2015 21:03:58 +0000
  Message-ID: \
    <CAAdUY-KN06pvCMy5bt3+Buk3yeKjf6n9iB2FaSTTOPpCqPwyhA@mail.gmail.com>

  https://lists.gnu.org/archive/html/emacs-devel/2015-11/msg01707.html

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