SCIPBackend: Faster copy, remove_constraint methods

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(from [#21003 comment:142](https://github.com/sagemath/sage/issues/21003#comment:142))

Depends on #21003

CC:  @mantepse @dimpase

Component: **linear programming**

Author: **Matthias Koeppe**

Reviewer: **Martin Rubey, ...**

_Issue created by migration from https://trac.sagemath.org/ticket/34890_

