Remove or fix some meaningless doctests for cmp()

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CC:  @mezzarobba

Component: **misc**

Author: **Marc Mezzarobba**

Branch/Commit: **[`6d2d496`](https://github.com/sagemath/sagetrac-mirror/commit/6d2d496b7adb3d650b3f881708961571e5a95f6a)**

Reviewer: **Frédéric Chapoton, Travis Scrimshaw**

_Issue created by migration from https://trac.sagemath.org/ticket/22376_

