Remove last uses of python3-simplejson in favor of json module

[cbosdo]

What does this PR change?

Remove the remaining uses of python3-simplejson in spacewalk-repo-sync and spacewalk-clone-by-date as reported in the user mailing list.
The standard json module is used instead.

GUI diff

No difference.

• DONE

Documentation

• No documentation needed: only internal and user invisible changes

• DONE

Test coverage

• No tests: package dependency

• DONE

Links

Fixes #
Tracks # add downstream PR, if any

• DONE

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[juliogonzalez]

This does not explain why this is needed, or what's going to be fixed.

[meaksh]

I'm not sure that we really want this. We explicitely dropped the simplejson some time ago, as we can use the json module from the Python3 std lib.

Why not simply removing this conflictive import? https://github.com/uyuni-project/uyuni/blob/master/python/spacewalk/satellite_tools/spacewalk-repo-sync#L25

[meaksh]

Thanks for the changes. LGTM 👍