Bug 15921: Do not include datatables js/css files twice

 Bug 15921: Do not include datatables js/css files twice

datatables.inc loads dataTables.buttons.min.js and buttons.colVis.min.js
since bug 15285: Update common files because the dom param now contains
'B'. The DT init fails with it does not know what 'B' means.

Test plan:
Test tables using buttons (columns visibility), they should work as
before this patch.

Tested with patron search and administration/currencies, works as expected.
Signed-off-by: Marc Véron <veron@veron.ch>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Brendan A Gallagher <brendan@bywatersolutions.com>