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

tests/test-runner/bin/zts-report.py

@@ -256,7 +256,6 @@ maybe = {
     'no_space/enospc_002_pos': ['FAIL', enospc_reason],
     'projectquota/setup': ['SKIP', exec_reason],
     'redundancy/redundancy_004_neg': ['FAIL', '7290'],
-    'removal/removal_resume_export': ['FAIL', '7894'],
     'reservation/reservation_008_pos': ['FAIL', '7741'],
     'reservation/reservation_018_pos': ['FAIL', '5642'],
     'rsend/rsend_019_pos': ['FAIL', '6086'],