Remove 500ns offset from GetSystemTicks()

This doesn't make any sense. The orignal idea might have been to
somehow be in the middle of 1ms, for whatever reason.

And it completely breaks, if we want to sleep 0ms.

Change-Id: I525b70c016876a96aa17edefe8c076b122ee2527

tools/source/datetime/ttime.cxx

@@ -433,7 +433,7 @@ sal_uInt64 tools::Time::GetSystemTicks()
         SAL_WARN("tools.datetime", "gettimeofday failed: " << e);
     }
     return static_cast<sal_uInt64>(tv.tv_sec) * 1000
-        + (static_cast<sal_uInt64>(tv.tv_usec) + 500) / 1000;
+        + static_cast<sal_uInt64>(tv.tv_usec) / 1000;
 #endif
 }