Bug 17825: Remove the use of AttributeTypeExists from opac-user.pl

Bug 17825: Remove the use of AttributeTypeExists from opac-user.pl

With the introducion of Koha::Object-based
Koha::Patron::Attribute::Type(s) there's no need for using this
function.

This patch replaces it with a regular ->search + ->count call.

To test:
- Have a user with checkouts logged into the OPAC on the opac-user.pl
page
- Set the SHOW_BCODE attribute to 'no' on the staff interface for the
user.
- Reload opac-user.pl
=> SUCCESS: The barcode is not shown for checked out items.
- Set the SHOW_BCODE atttribute to 'yes' on the staff interface for the
user.
- Reload opac-user.pl
=> SUCCESS: The barcode is shown for checked out items.
- Delete the Patron attribute type SHOW_BCODE
- Reload opac-user.pl
=> SUCCESS: No barcode is shown for checked out items
- Sign off :-D

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>