bug 3212: MARC21 bib leader plugin sets Leader/09 to 'a'

bug 3212: MARC21 bib leader plugin sets Leader/09 to 'a'

Changed the plugin so that the form for editing the MARC21
leader now sets postion 9 (character coding) to 'a' (UCS/Unicode)
and no longer gives the cataloger the option to change it.  Koha
uses UTF-8 exclusively for MARC records, and a number of bugs
can be triggered by storing MARC21 bib and authority record
whose Leader/09 is anything other than 'a'.  Since internal
routines are increasingly ensuring that the MARC21 Leader/09 is
set to 'a', it doesn't make sense to give catalogers the idea
that it's possible (or a good idea) to set the Leader/09 to any
other value.

Signed-off-by: Galen Charlton <galen.charlton@liblime.com>
Signed-off-by: Henri-Damien LAURENT <henridamien.laurent@biblibre.com>