CLEANUP: peers: remove unused "proto" and "xprt" struct members

CLEANUP: peers: remove unused "proto" and "xprt" struct members

peer->proto and peer->xprt struct members are now pure legacy: they are
only set during parsing but never used afterwards.

This is due to commit 02efedac ("MINOR: peers: now remove the remote
connection setup code") which made some cleanup in the past, but the
unused proto and xprt members were probably left unused by mistake.

Since we don't have valid uses for them, we remove them.

Also, peer_xprt() helper function was removed since it was related to
peer->xprt struct member.