Preemptive deletion of videos when different quality is set to Download

[samuelechu]

It looks like the function call to isVideoStored() in
https://github.com/samuelechu/YoutubePlaylist-Downloader/blob/master/Music%20Player/DownloadManager.swift#L60

also deletes the video in question if the quality is not the same as the desired quality set.
If a 360p video is set to be downloaded and a 720p video of it exists, the 720p video is deleted before the download even starts. This is potentially bad if the user downloads some large playlist frequently and forgets to change the quality in settings.

Deleting a video in a function named in a way that only a check is expected is bad design