Bug 15481: Remove dead code in datatables.js

 Bug 15481: Remove dead code in datatables.js

This removes the following functions that are not used anywhere:
- dt_add_rangedate_filter
- dt_overwrite_string_sorting_localeCompare
- replace_html
- replace_html_date

Test plan:
1. Search for calls to the above functions in the code. There should be
   none.
2. Check that existing tables using DataTables are not broken.

Signed-off-by: Marc Véron <veron@veron.ch>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Brendan A Gallagher <brendan@bywatersolutions.com>