Remove deprecated PowerSeries._floordiv_

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Deprecated in #20062.

Component: **algebra**

Author: **Jeroen Demeyer**

Branch/Commit: **[`8820e7b`](https://github.com/sagemath/sagetrac-mirror/commit/8820e7b2e6cac3b56f2bc5b3d334057eee583373)**

Reviewer: **David Roe**

_Issue created by migration from https://trac.sagemath.org/ticket/24608_

