Remove unused methods in SampleVNF

Remove unused methods in SampleVNF

The following private methods are unused in
yardstick/network_services/vnf_generic/vnf/sample_vnf.py:SampleVNF

* _build_ports(self)
* _get_route_data(self, route_index, route_type)
* _get_port0localip6(self)
* _get_port1localip6(self)
* _get_port0prefixlen6(self)
* _get_port1prefixlen6(self)
* _get_port0gateway6(self)
* _get_port1gateway6(self)

They are being removed as:
* _get_port* functions are already implemented and used in MultiPortConfig
* _get_route_data is an auxillary functions used by the previous ones
* _build_ports is implemented in the helpers

JIRA: YARDSTICK-1122
Change-Id: Ie7a706cd87b610ea56f89214118a02e612a70dc0
Signed-off-by: Emma Foley <emma.l.foley@intel.com>
(cherry picked from commit 50bcd3e477bc29abb43ed73643d551ff55a73403)