Bug 11655: koha-translate --list no longer shows 'en'

Bug 11655: koha-translate --list no longer shows 'en'

'en' is not a translation and it doesn't make any sense to return it as
part of the installed translations or the available ones.

It becomes relevant to remove it as users upgrading their installed
translations end up grepping the output to remove 'en' like in:

for lang in $(koha-translate --list | grep -v -x -e 'en') ; do
  koha-translate --update $lang
done

instead of simply issuing the koha-translate --list command.

To test:
- Run
  ./koha-translate --list
  ./koha-translate --list --available
=> It shouldn't return 'en' in the result list.

Sponsored-by: Universidad Nacional de Cordoba
Signed-off-by: Chris Cormack <chris@bigballofwax.co.nz>
Signed-off-by: Jonathan Druart <jonathan.druart@biblibre.com>
Signed-off-by: Galen Charlton <gmc@esilibrary.com>
(cherry picked from commit 6aa021896827e846e58f88a8c5387df122688c53)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>