sagemathgh-39771: Remove conda-ci
    
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The old setuptools-sagelib on top of conda installation is now surpassed
by meson, so there is no point in having tests for this way of
installation. The documentation still needs to be revised, either as
part of or as follow-up to sagemath#39515.

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### ⌛ Dependencies

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URL: sagemath#39771
Reported by: Tobias Diez
Reviewer(s): Dima Pasechnik

Author: Release Manager