ZTS: Fix removal_resume_export

ZTS: Fix removal_resume_export

This change simplify the test case removing part of the logic which was
introducing a race condition and thus causing spurious failures: we use
attempt_during_removal() from removal.kshlib instead which has been
observed to be more stable.

Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed-by: John Kennedy <john.kennedy@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: loli10K <ezomori.nozomu@gmail.com>
Closes #7894
Closes #7913