Bug 17804: Remove some modules from showdiffmarc.pl

Remove DBI, LibXML, LibXSLT.
Add CGI's -utf8 flag.
Few whitespace changes.

Test plan:
[1] Export an existing record to marcxml.
[2] Edit the file, make some small changes.
[3] Import it again, use a matching rule.
[4] Check the diff on Manage staged. (Here is showdiffmarc.pl)

Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>