Do not show user sync tasks in the Channels tasks

[jredrejo]

Summary

In a Learner Only Device, the syncing of users tasks were appearing in the channels importing tasks, showing an ugly TaskPanel.undefined message

This PR filters this kind of tasks to avoid their appearance in this window

References

Closes: #8393

Reviewer guidance

After creating a LOD, check that no odd tasks message appear in the Channels tab when the users are synced in the background

Testing checklist

• Contributor has fully tested the PR manually
• If there are any front-end changes, before/after screenshots are included
• Critical user journeys are covered by Gherkin stories
• Critical and brittle code paths are covered by unit tests

PR process

• PR has the correct target branch and milestone
• PR has 'needs review' or 'work-in-progress' label
• If PR is ready for review, a reviewer has been added. (Don't use 'Assignees')
• If this is an important user-facing change, PR or related issue has a 'changelog' label
• If this includes an internal dependency change, a link to the diff is provided

Reviewer checklist

• Automated test coverage is satisfactory
• PR is fully functional
• PR has been tested for accessibility regressions
• External dependency files were updated if necessary (yarn and pip)
• Documentation is updated
• Contributor is in AUTHORS.md