Apparently Vista's printf-like functions don't support %n by default.

We could explicitly re-enable %n, but we only use it in one place, so take
the path of least resistance and remove that single instance. This stops
dupvprintf() getting stuck in a loop (a behaviour that's caused by a workaround
for a broken libc).

<http://msdn.microsoft.com/en-us/library/ms175782(VS.80).aspx>

git-svn-id: svn://svn.tartarus.org/sgt/putty@8030 cda61777-01e9-0310-a592-d414129be87e