Bug 29040: Remove warning from the itemtype edit view

Bug 29040: Remove warning from the itemtype edit view

Use of uninitialized value $interface in concatenation (.) or string at /kohadevbox/koha/C4/Languages.pm line 121.

We are expecting "intranet" or "opac, here we sent undef which raises a
warning.
For no change in behaviour we can send "both" (or whatever different
than "intranet" and "opac").

Test plan:
Hit /cgi-bin/koha/admin/itemtypes.pl?op=add_form&itemtype=BK
Notice that the warning does no longer appear in the logs with the patch
applied.

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>