Duplicate THREADINFO allocated by UserInitialize

We get this on every boot:

(win32ss/user/ntuser/main.c:384) err: PsGetThreadWin32Thread returned non-NULL thread info!!

The message indicates that when trying to allocate a THREADINFO structure in AllocW32Thread, there was already one present; the result being that we overwrite and leak the old one.
This is done by InitThreadCallback, which gets called from user32's thread attach. However we also call it from (Nt)UserInitialize, which seems to be redundant.