`BgpPeer::interface_name` isn't faithfully persisted in the db - we should remove it

[jgallagher]

This originally came up on #10086 (comment). When we record a BgpPeer structure in the db, we assign its interface_name in the db to the link_name of its enclosing BgpPeerConfig, ignoring the BgpPeer::interface_name field. @rcgoodfellow notes this code is going to change significantly soon, but that this field is somewhat vestigial:

I believe this field was intended to support breakout configurations and vlan interfaces. We've not had a need to implement breakouts, and VLANs wound up being implemented in a way that does not involve explicit interfaces. This field can likely be removed.