Revert "Add simple check for mergeable status in find_candidates()"

This reverts commit 920bb3718a2818bbf9ef86414d45ff343dd94076.

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Transient error noticed by @mtpc during review of #152 was noticed in http://ci.openmicroscopy.org/job/OMERO-5.0-merge-integration/203/consoleFull again. As this could seriously harm our review workflow esp. close to a release. I propose to revert the commit and re-release a new version of scc.