Remove unused code for xmpp ecmp from client to server

[Nischal Sheth]

We added support for ecmp next hops in xmpp routes advertised from client
to server. This was done with the goal of supporting multi-homing with 2
different L3 interfaces on the compute node.

The multiple L3 interface feature was never productized and our thinking
and design for it has now evolved. We will support multiple L3 interfaces
by introducing the notion of a loopback address for the compute node, much
the same as what is done for regular routers.

Hence we can remove some vestigial code.

[OpenContrail Admin (ci-admin-f)]

Review in progress for https://review.opencontrail.org/30217
Submitter: Nischal Sheth (<email address hidden>)

[OpenContrail Admin (ci-admin-f)]

Reviewed: https://review.opencontrail.org/30217
Committed: http://github.org/Juniper/contrail-controller/commit/fced5db727dbe6e1ec2a8aa595ba6a5b87690254
Submitter: Zuul (<email address hidden>)
Branch: master

commit fced5db727dbe6e1ec2a8aa595ba6a5b87690254
Author: Nischal Sheth <email address hidden>
Date: Tue Apr 4 17:34:46 2017 -0700

Remove xmpp ecmp support in client to server direction

Following changes are implemented:

- Simplify BgpTable::Input as it doesn't have to delete paths implicitly
- Do not use the path_id to find a BgpPath added by BGP_XMPP peer
- BgpTable::RequestData now contains a single nexthop instead of a list
- Remove receive code to handle multiple nexthops in bgp_xmpp_channel.cc
- Remove tests related to xmpp ecmp
- Modify mock agent to no longer support route add with ecmp nexthops
- Remove test::NextHops from mock agent
- Modify tests to use singular test::NextHop instead of test::NextHops
- Modify path_resolver_test to no longer rely on xmpp ecmp support

Change-Id: I2a38f472a33c731294772b1b22879b75eb05291a
Closes-Bug: 1680589