checkbox-touch: remove 'forward' icon from button on selection page

This patch removes icon from Continue button on selection page component.

Fixes: https://bugs.launchpad.net/checkbox-touch/+bug/1382966
Signed-off-by: Maciej Kisielewski <maciej.kisielewski@canonical.com>
"automatic merge by tarmac [r=zkrynicki][bug=1382966][author=kissiel]"