chore: remove deprecated flux code (to be released as an executor plu…

…gin) (#2599)

### Description

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### QC
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* [x] The PR contains a test case for the changes or the changes are
already covered by an existing test case.
* [x] The documentation (`docs/`) is updated to reflect the changes or
this is not necessary (e.g. if the change does neither modify the
language nor the behavior or functionalities of Snakemake).