tst_QFile: remove unused variable on Windows

Removes a warning in the build.

Pick-to: 6.4
Change-Id: I07ec23f3cb174fb197c3fffd17215c40b40333cb
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>

tests/auto/corelib/io/qfile/tst_qfile.cpp

@@ -1557,7 +1557,6 @@ void tst_QFile::copyFallback()
 #if defined(Q_OS_WIN)
 static QString getWorkingDirectoryForLink(const QString &linkFileName)
 {
-    bool neededCoInit = false;
     QString ret;
 
     QComHelper comHelper;