Remove 1 px border around Start Center

It leaks workspace background color (behind the sheet in Writer etc.), which
looks bad as it's much darker now under Windows and OS X.

Change-Id: I29034be86d3cf5537384b325fc6734b4dc05f077
Reviewed-on: https://gerrit.libreoffice.org/10521
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>