Bug 9988: Remove further references to dontmerge

Bug 9988: Remove further references to dontmerge

[1] The preference was sent to HEA. We can now send both AuthorityMergeMode
    as well as AuthorityMergeLimit.
[2] A comment in authorities/merge.pl is removed. Note that a subsequent
    patch will modify and test the cron job.
[3] Script misc/batchRebuildItemsTables.pl temporarily enabled dontmerge.
    This is equivalent to setting the mergelimit to zero.
    The function defnonull is no longer needed. (If the pref was NULL,
    we restore that value. Sub merge won't mind.)

Test plan:
[1] Run t/db_dependent/UsageStats.t
[2] Run misc/batchRebuildItemsTables.pl -t
    This just ensures you it still compiles; the changes speak for itself.
[3] Now git grep on dontmerge.
    You should only find hits in atomicupdate and misc/translator/po.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Marc Véron <veron@veron.ch>
Signed-off-by: Jacek Ablewicz <abl@biblos.pk.edu.pl>
Signed-off-by: Julian Maurice <julian.maurice@biblibre.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>