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

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.