Bug 36395: Remove useless fetch in admin/marc_subfields_structure.pl

Bug 36395: Remove useless fetch in admin/marc_subfields_structure.pl

Since (a long time ago!)
  commit 62aa3f4292d83bc9304e296de5f0830e28510492
  Bug 17216: Use Koha::AVC from mss.pl

This $sth2 is never fetched, we are retrieving the AV cat using
Koha::AV::Categories

Sponsored-by: The Research University in the Helmholtz Association (KIT)
Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>
(cherry picked from commit 4158783eb284cf5f89266d3251f6d647200c0f01)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>