ldpd: remove the interface vty node

ldpd: remove the interface vty node

ldpd uses a hierarchical configuration model where all commands are
defined inside the "mpls ldp" node and its subnodes. The idea is to keep
all LDP configuration in a single place to keep things simple. With that
said, we can remove the "config-if" node from ldpd because we already
have a separate node ("config-ldp-af-if") for LDP-related interface
specific commands.

Example:
vtysh(config)# mpls ldp
vtysh(config-ldp)# address-family ipv4
vtysh(config-ldp-af)# interface eth1
vtysh(config-ldp-af-if)# discovery hello ?
  holdtime  Hello holdtime
  interval  Hello interval

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>