Refactor: attrd: remove dead code checking node name vs. ID

attrd's ATTRD_OP_PEER_REMOVE handling previously checked whether the
specified target was a node name or node ID.

However, none of the existing callers use this capability, and even if
anything did, attrd_peer_remove() would return without doing anything,
so there was no way the node ID code could be effective.

Specifically, crm_node puts the node name and node ID in the appropriate
XML fields, so the node name field will never have the ID; crmd only calls the
op for remote nodes, which don't have a numeric ID; and attrd's peer change
callback doesn't uncache the node, so attrd_peer_remove() won't use the ID in
that case.