Simplify checking of the SHM/SEM Unix key file's existence

We don't need to stat(2) the file to check if it exists before
creating it. We're about to open(2) with O_CREAT|O_EXCL, which will
not recreate it if it existed.

Change-Id: I2d2176054e7776a32a4520832ee104b9c4ccf748
Reviewed-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>
Reviewed-by: Richard J. Moore <rich@kde.org>

src/corelib/kernel/qsharedmemory_unix.cpp

@@ -151,9 +151,6 @@ key_t QSharedMemoryPrivate::handle()
   */
 int QSharedMemoryPrivate::createUnixKeyFile(const QString &fileName)
 {
-    if (QFile::exists(fileName))
-        return 0;
-
     int fd = qt_safe_open(QFile::encodeName(fileName).constData(),
             O_EXCL | O_CREAT | O_RDWR, 0640);
     if (-1 == fd) {