Bug 28281: Remove double decoding when importing installation data

Bug 28281: Remove double decoding when importing installation data

The YAML::XS::Load and YAML::XS::LoadFile functions already decode the
YAML to Perl objects/strings, therefore decoding the output yet
another time is not needed and causes an error to happen.

To test:

1. Go through the following steps and notice without this patch the
error happens and with this patch it doesn't:

$ koha-mysql kohadev < installer/data/mysql/kohastructure.sql
$ restart_all
$ ./translate update pl-PL
$ ./translate install pl-PL
$ restart_all
 --> Go to the Koha web installer page and select pl-PL as the installer
 language, click through it and notice it gives following error at the
 MARC21 step:

 Cannot decode string with wide characters at /usr/lib/x86_64-linux-gnu/perl/5.24/Encode.pm line 202

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>