Bug 12320: Remove deprecated construct to delete cookie showColumns

Bug 12320: Remove deprecated construct to delete cookie showColumns

  $.cookie('foo', null);
is deprecated and should be replaced with
  $.removeCookie('foo');

This patch replaces the occurrences for the "showColumns" cookie.

Before this patch, there was a bug on the batchmod tools.
To reproduce the issue:
1/ Go on the Batch item modification tool
2/ Fill the textarea with barcodes and submit
3/ Click on some column names (to create the cookie)
4/ Click on 'Show all columns" (to set the cookie to null)
5/ Don't submit and repeat steps 1 & 2
6/ You should see a js error:
  Error: Syntax error, unrecognized expression: :nth-child
    ...break;q=a}return s},m.error=function(a){throw new Error("Syntax error, unrecogni...

Test plan:
Confirm the issue has gone away and there is no regression on the column
selection

Signed-off-by: Bernardo Gonzalez Kriegel <bgkriegel@gmail.com>
No js error, no regressions, no errors

Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>