remove networkError handling

[lmoj]

if a fragment doesnt exist in a hls playlist such as:

#EXTM3U
#EXT-X-VERSION:7
#EXT-X-TARGETDURATION:8
#EXT-X-MEDIA-SEQUENCE:0
#EXTINF:7.5,
/doesnt_exist.ts

_onNetworkError will call startLoad again on the 404, and the next, etc. causing a ddos of the resource. hls.js handles the retries on its own.