Bug 9988: [QA Follow-up] Satisfy QA issues

Bug 9988: [QA Follow-up] Satisfy QA issues

[1] See comment102. Moved sub reporting_tag_xml to MergeRequest.pm.
    Adjusted t/db_dependent/Koha/Authorities.t accordingly.
    This resolves the C3 inconsistent hierarchy errors.
[2] Removed empty POD section Instance Methods from MergeRequests.
    This resolves the POD error in comment102 point 2.
[3] Include a tag 100 for UNIMARC in reporting_tag_xml to resolve an error
    on encoding in MARC::File::XML. Subtest for oldmarc and subtest for
    reporting_tag_xml adjusted accordingly.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Julian Maurice <julian.maurice@biblibre.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>