Bug 15918 - Obsolete file datatables-strings.inc can be removed

Bug 15918 - Obsolete file datatables-strings.inc can be removed

The include file datatables-strings.inc is no longer necessary. Its
contents are now in datatables.inc. This patch corrects two references
in comments to datatables-strings.inc (replacing it with datatables.inc)
and removes the obsolete file.

To test, apply the patch and load any page with a DataTables-enabled
table configured to show pagination buttons, e.g. Saved Reports.
Pagination labels like "Next," "Previous," and "Showing X of X..."
should be correct.

Search the codebase to confirm that no references remain for
datatables-strings.inc.

Signed-off-by: Marc Véron <veron@veron.ch>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>