Bug 11616: remove jQuery tablesorter plugin

DataTables has replaced the tablesorter plugin for client-side sorting
of tables in Koha.  There is no table using this plugin, so this patch
removes it and remaining references to it.

REVISED TEST PLAN
-----------------
1) Apply the patch
2) Home -> Koha administration -> Authorized values
3) Change the 'Show category:' drop down value, and play with
    the sorting of columns.
   -- should sort as expected.
4) Search the catalogue -> look for a biblio with high circulation
5) Click one of the name links.
6) Click the Items tab on the left.
7) Scroll down and click the (View item's checkout history)
    link in the History area.
   -- There was no sorting prior to the patch, so afterwards
      it should display the same.
8) git grep -i tablesorter
   -- Only a reference in staff-global.css and release texts.

Signed-off-by: Mark Tompsett <mtompset@hotmail.com>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Galen Charlton <gmc@esilibrary.com>
(cherry picked from commit 166720982e59a929aac9191d89af57a57545ff15)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>
Bug 11616: (follow-up) remove tablesorter CSS

This patch removes CSS rules that applied only to use of the
jQuery tablesorter plugin, which is now gone.

To test:

[1] Verify that git grep -l headerSort does not turn up
    any results.

Signed-off-by: Galen Charlton <gmc@esilibrary.com>
(cherry picked from commit 1a5da08342182c739f2f5b790038c6ff04c748ea)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>