As this fiber is made system in the commit bf620650 ("box: finish
client fibers on shutdown") we don not need the existing protection from
cancelling. So first remove it. Now make it managed on shutdown.

Note that we may have issues as we finish this fiber too early. The
tasks scheduled but not executed at this moment will never be executed.
So the tasks that be scheduled after fiber is finished. Now when we
don't use worker fiber for swim gc this will not cause leaks. And
leaking fd on Tarantool shutdown in fio is not a problem.

Closes #9722

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