user32/tests: Remove unused variables (Clang)

[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=10592

Your paranoid android.


=== w7u (32 bit msg) ===
msg.c:12105: Test failed: unexpected hwnd 0012013C
msg.c:12127: Test failed: bad time 310a2/310b2