Bug 14588: Simplify merge_ajax.pl

Bug 14588: Simplify merge_ajax.pl

This script does not need a few modules:
    IO::File; CGI::Session; C4::UploadedFile.
Warnings can be enabled (with Modern::Perl).
Indirect object syntax replaced for CGI and CGI::Cookie.
Moved the $reply line upwards (not needed twice anymore).

Test plan:
[1] Log in as staff user. Run the URL cataloguing/merge_ajax.pl.
    You should see the JSON reponse in the browser.
[2] Logout. Run the URL again. Blank screen.
[3] Go to addbooks.pl (Cataloging Search). Search something, select two
    biblios and click Merge selected, etc.

Signed-off-by: Joonas Kylmala <j.kylmala@gmail.com>
Signed-off-by: Mark Tompsett <mtompset@hotmail.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Tomas Cohen Arazi <tomascohen@unc.edu.ar>