removed outdated IsDebuggerPresent declaration

This piece of code seems to retain compatibility with some windows
versions before 2000. Today it's not relevant anymore and only
produces a warning C4273 because of the conflicting prototype. In
VC9 the declaration is present in winbase.h:4142, but available
is it already in winxp. Here's the doc link
http://msdn.microsoft.com/en-us/library/windows/desktop/ms680345(v=vs.85).aspx