Bug 17645 - Remove obsolete interface customization images

Bug 17645 - Remove obsolete interface customization images

This patch removes three files which are obsolete following the removal
of the OPAC prog template.

To test, apply the patch and confirm that these files no longer exist in
misc/interface_customization:

- koha3-opac-button-background.png
- koha3-opac-button-background.psd
- koha3-opac-button-background.svg

Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Jonathan Druart <jonathan.druart@biblibre.com>
Signed-off-by: Mason James <mtj@kohaaloha.com>