Bug 9508: Standardize the dateformat value from C4::Auth

Bug 9508: Standardize the dateformat value from C4::Auth

- the dateformat value is send to all templates (from
  C4::Auth::get_template_and_user)
- remove all assignment of dateformat in all .pl files

- Remove "all" occurrences (those I found!) of dateformat_*
From now the only way to get the date format is a string comparaison
(dateformat == "metric")

Checked with the command:
  git grep "\(dateformat_us\|dateformat_metric\|dateformat_iso\)" | grep
  -v translator

Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Tested all the datepickers I could find, looks good.

Signed-off-by: Paul Poulain <paul.poulain@biblibre.com>
Signed-off-by: Galen Charlton <gmc@esilibrary.com>
(cherry picked from commit 537c66403855437d2ce52eeb758fc009c7105633)