Change the “X” icons for removing documents in Start Center

Change-Id: Icb118ef49d46b983ff535c29e5b82a2da61700e1
Reviewed-on: https://gerrit.libreoffice.org/12144
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>