QNetworkDiskCache -  fix leak of file descriptor

When the caches is deleted, the open files are deleted without closing action.
The file descriptor is remaining until the process is terminated.

Change-Id: If85519d173d05548ddf3273c85800441887199e2
Reviewed-by: jungo kim <jungo.kim@lge.com>
Reviewed-by: Peter Hartmann <phartmann@blackberry.com>