Bug 21234: Remove our .clearfix class in favor of Bootstrap's

Bug 21234: Remove our .clearfix class in favor of Bootstrap's

This patch removes the ".clearfix" declaration from staff-global.scss so
that Bootstrap's built-in clearfix class will be used.

To reproduce this issue, see Administration -> Currencies and exchange
rates. The table of currencies does not appear below the table buttons.

Apply the patch and regenerate CSS
(https://wiki.koha-community.org/wiki/Working_with_SCSS_in_the_OPAC_and_staff_client).

Clear the browser cache if necessary and reload the Currencies and
exchange rates page. The table should appear in the correct position.

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>