Bug 29556: MARC21slim2MODS.xsl broken by duplicate template name "part"

Bug 29556: MARC21slim2MODS.xsl broken by duplicate template name "part"

This patch removes a redundant section from MARC21slim2MODS.xsl which
was breaking the option to use unAPI in the OPAC using MODS format.

To test, apply the patch and view a record in the OPAC using unAPI,
e.g.:

/cgi-bin/koha/unapi?id=koha:biblionumber:1&format=MODS

You should see an XML file where previously there would have been an
error.

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>