Bug 35935: Ensure login branch will be used after incorrect login

Bug 35935: Ensure login branch will be used after incorrect login

If a different branch is selected after an incorrect login, the previous
branch will be used.

To recreate:
* login with foo/bar, select CPL => FAIL
* login with koha/koha, select another branch => OK but CPL is picked!

It was caused by a dup of "branch" in CGI param list (and first was
picked).

This patch patch also removes "koha_login_context" to not have it twice.
You can also open the source of the page to confirm that form#loginform
contains "branch" and "koha_login_context" in hidden inputs.

Signed-off-by: Magnus Enger <magnus@libriotech.no>
Tested in KTD. Works as advertised.
Signed-off-by: Julian Maurice <julian.maurice@biblibre.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>