Bug 20433: Remove unused Mozilla Persona image file

Bug 20433: Remove unused Mozilla Persona image file

This patch removes an unused image file from the OPAC which was added
when we added Mozilla Persona authentication but wasn't removed when
Mozilla Persona went away.

To test, apply the patch and confirm that the image no longer exists:

koha-tmpl/opac-tmpl/bootstrap/images/sign_in_green.png

Signed-off-by: Mark Tompsett <mtompset@hotmail.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>