CLEANUP: backend: remove impossible case of round-robin + consistent hash

In 1.4, consistent hashing was brought by commit 6b2e11be1 ("[MEDIUM]
backend: implement consistent hashing variation") which took care of
replacing all direct calls to map_get_server_rr() with an alternate
call to chash_get_next_server() if consistent hash was being used.

One of them, however, cannot happen because a preliminary test for
static round-robin is being done prior to the call, so we're certain
that if it matches it cannot use a consistent hash tree.

Let's remove it.
diff --git a/src/backend.c b/src/backend.c
index 2041c43..f5fec1d 100644
--- a/src/backend.c
+++ b/src/backend.c
@@ -679,10 +679,9 @@
 		case BE_LB_LKUP_CHTREE:
 		case BE_LB_LKUP_MAP:
 			if ((s->be->lbprm.algo & BE_LB_KIND) == BE_LB_KIND_RR) {
+				/* static-rr (map) or random (chash) */
 				if ((s->be->lbprm.algo & BE_LB_PARM) == BE_LB_RR_RANDOM)
 					srv = get_server_rnd(s, prev_srv);
-				else if ((s->be->lbprm.algo & BE_LB_LKUP) == BE_LB_LKUP_CHTREE)
-					srv = chash_get_next_server(s->be, prev_srv);
 				else
 					srv = map_get_server_rr(s->be, prev_srv);
 				break;