nixosTests.nscd: drop _gateway lookups

This has shown to be flaky in the VM test, at least when running on
the aarch64 ofborg builder(s).

I assume it's some flakyness in systemd-networkd not being fully up, or
at least not up to the point that it properly replies to the _gateway
request.

This part of the test is supposed to test external (non-glibc) nss
module lookup for the host database works, which is already sufficiently
covered in the previous checks (for *.localhost). Drop these redundant
checks. We're not integration-testing networkd here.

nixos/tests/nscd.nix

@@ -84,21 +84,6 @@ in
               assert "::1" in output
               assert "127.0.0.1" not in output
 
-              # ahosts
-              output = machine.succeed("getent ahosts _gateway")
-
-              # returns something like the following:
-              # 10.0.2.2        STREAM _gateway
-              # 10.0.2.2        DGRAM
-              # 10.0.2.2        RAW
-              # fe80::2         STREAM
-              # fe80::2         DGRAM
-              # fe80::2         RAW
-
-              # Verify we see both ip addresses
-              assert "10.0.2.2" in output
-              assert "fe80::2" in output
-
       start_all()
       machine.wait_for_unit("default.target")