Bug 14975: Fine grace period label is incorrect

Bug 14975: Fine grace period label is incorrect

The column heading for grace period is "Fine grace period (day)" but the
grace period works based on the unit selected ( hours or days ). The
"(day)" part should be removed as it caused confusion for librarians.

Test Plan:
1) Apply this patch
2) Go to the circ rules editor
3) Note the header and footer columns for grace period no longer
   have "(days)" at the end.

Signed-off-by: Marc Véron <veron@veron.ch>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>