rpcrt4/tests: Remove an unnecessary and confusing message.

[Marvin]

Hi,

While running your changed tests on Windows, I think I found new failures.
Being a bot and all I'm not very good at pattern recognition, so I might be
wrong, but could you please double-check?
Full results can be found at
https://testbot.winehq.org/JobDetails.pl?Key=25500

Your paranoid android.


=== w864 (32 bit rpc) ===
rpc.c:812: Test failed: GUID does not appear to contain a MAC address: {f03f0f60-7046-11e6-be78-e71666c2e04a}

=== w864 (64 bit rpc) ===
rpc.c:812: Test failed: GUID does not appear to contain a MAC address: {ab19995e-7047-11e6-be78-e71666c2e04a}