Bug 18980: Remove js function Check from opac-shelves

Bug 18980: Remove js function Check from opac-shelves

This function only serves to check if the shelf name is not empty. It
does not even work since it incorrectly refers to f.addshelf (while it
should test #shelfname).
We can solve this and improve consistency by doing the same as in the
staff template with html5 required.

Test plan:
[1] Without this patch it was possible to add a list without a name in
    OPAC (with js error: TypeError: f.addshelf is undefined).
[2] With this patch, verify that you cannot do this anymore.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Lee Jamison <ldjamison@marywood.edu>
Signed-off-by: Marc Véron <veron@veron.ch>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>