Bug 29629: Remove two unused MODS XSLT stylesheets

Bug 29629: Remove two unused MODS XSLT stylesheets

Follow-up of bug 29556. With thanks to Owen for checking.

> koha-tmpl/intranet-tmpl/prog/en/xslt/MARC21slim2MODS3.xsl
> koha-tmpl/intranet-tmpl/prog/en/xslt/MARC21slim2MODS32.xsl
> These two files appear to be unused. I could only find references to them in .po files.

Test plan:
git grep MARC21slim2MODS32
git grep MARC21slim2MODS3
Skip the misc/translator/po references. Note in the second grep that
you only see that MODS3-1 is used in unapi and C4::Record.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>