Remove unused code for BGP inet6 NLRI encoding and decoding

Couple of other minor changes:

- BuildProtoPrefix and BuildBgpProtoNextHop are not pure virtual anymore
- Do not use namespace using-directive in rtarget_route.cc

Change-Id: Ib5c81b520b701bb2fe772792d839683be4d3db7b
Merge "Remove unused code for BGP inet6 NLRI encoding and decoding"