CLEANUP: backend: drop checks for OBJ_TYPE_PROXY in connect() code

CLEANUP: backend: drop checks for OBJ_TYPE_PROXY in connect() code

In tcp_connect_server(), uxst_connect_server(), and quic_connect_server(),
we can no longer see obj_type(conn->target) == OBJ_TYPE_PROXY so let's
drop that code. This implies that srv may no longer be NULL so we can
drop these checks as well.