removal of some unused imports about string conversion

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as this may help to reduce compilation warnings

Component: **refactoring**

Author: **Frédéric Chapoton**

Branch/Commit: **[`fc44813`](https://github.com/sagemath/sagetrac-mirror/commit/fc44813022d61df0d9a6763779d6c71668794df3)**

Reviewer: **Travis Scrimshaw**

_Issue created by migration from https://trac.sagemath.org/ticket/34844_

