Bug 35329: Remove fixedHeader

Bug 35329: Remove fixedHeader

If fixedHeader is set for the result table in the modal, the header will
still appear (quite randomly) on the main view.

To recreate the problem:
Go to /cgi-bin/koha/members/memberentry.pl
Click Add guarantor
Search
Close the modal
Scroll down
=> The header of the table will be displayed randomly at the top of the
page

Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>