is_router_version_good_for_possible_guard() no longer needed

The function returns true for relays that are at least these versions:

    /* CVE-2011-2769 was fixed on the relay side in Tor versions
     * 0.2.1.31, 0.2.2.34, and 0.2.3.6-alpha. */
    tor_assert(tor_version_parse("0.2.1.31",
                                 &first_good_0_2_1_guard_version)>=0);
    tor_assert(tor_version_parse("0.2.2.34",
                                 &first_good_0_2_2_guard_version)>=0);
    tor_assert(tor_version_parse("0.2.3.6-alpha",
                                 &first_good_later_guard_version)>=0);

yet all relays are at least those versions:

  /* Versions before Tor 0.2.3.16-alpha are too old to support, and are
   * missing some important security fixes too. Disable them. */
  if (platform && !tor_version_as_new_as(platform,"0.2.3.16-alpha")) {
    if (msg)
      *msg = "Tor version is insecure or unsupported. Please upgrade!";
    return FP_REJECT;
  }