hlssink2: Respect requested max-files property for decision on removing old fragments from disk

hlssink2 defined "max-files" property to decide the maximum number of fragments which should be stored in disk. But we've not used the property. Instead, the size has been maintained by "playlist-length". Since "max-files" and "playlist-length" have different meaning, the decision should be done by "max-files" property.

For example, an user might want expose only 3 fragments via playlist but might want to keep more files than 3 in disk.