[dart:io] Remove error number print from windows process spawning

The error number is already reported in the ProcessException,
and the log print might pollute the result of GetLastError().

TEST=N/A
Change-Id: Ia280d02525e688c1891bb1ff3131b1569a35083e
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/185203
Commit-Queue: Zach Anderson <[email protected]>
Reviewed-by: Alexander Aprelev <[email protected]>

Author: zanderso

runtime/bin/process_win.cc

@@ -564,7 +564,6 @@ class ProcessStarter {
         reinterpret_cast<STARTUPINFOW*>(&startup_info), &process_info);
 
     if (result == 0) {
-      Syslog::PrintErr("CreateProcessW failed %d\n", GetLastError());
       return CleanupAndReturnError();
     }