[twitch] remove rechat.twitch.tv as a subtitle source

[vgsdi]

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---

Description of your pull request and other information

I've noticed that youtube-dl returns an url for rechat.twitch.tv as a subtitle, but the domain doesn't even exist anymore. It looks like twitch shut down this service back in october. This also causes some errors when mpv uses youtube-dl to open twitch VODs.

testing with youtube-dl:

>youtube-dl.py --verbose --write-sub https://www.twitch.tv/videos/235151000
[debug] System config: []
[debug] User config: []
[debug] Custom config: []
[debug] Command-line args: [u'--verbose', u'--write-sub', u'https://www.twitch.tv/videos/235151000']
[debug] Encodings: locale cp1250, fs mbcs, out cp852, pref cp1250
[debug] youtube-dl version 2018.03.10
[debug] Python version 2.7.12 (CPython) - Windows-7-6.1.7601-SP1
[debug] exe versions: ffmpeg N-89127-g8f4702a93f, ffprobe N-89127-g8f4702a93f
[debug] Proxy map: {}
[twitch:vod] 235151000: Downloading vod info JSON
[twitch:vod] 235151000: Downloading vod access token
[twitch:vod] 235151000: Downloading m3u8 information
[debug] Default format spec: bestvideo+bestaudio/best
[info] Writing video subtitles to: Holi _3  La Comarca !sorteo-v235151000.rechat.json
WARNING: Unable to download subtitle for "rechat": Unable to download webpage: <urlopen error [Errno 11004] getaddrinfo failed> (caused by URLError(gaierror(11004, 'getaddrinfo failed'),))
[debug] Invoking downloader on u'https://vod099-ttvnw.akamaized.net/df0e8d367becf8ccaf6b_duendepablo_27810724960_806840479/chunked
/index-muted-I7UUYYDRM4.m3u8'
[hlsnative] Downloading m3u8 manifest

ERROR: Interrupted by user

mpv:

>mpv https://www.twitch.tv/videos/235151000
Playing: https://www.twitch.tv/videos/235151000
[ffmpeg] tcp: Failed to resolve hostname rechat.twitch.tv: No address associated with hostname
Failed to open https://rechat.twitch.tv/rechat-messages?video_id=v235151000&start=1520175636.
Can not open external file https://rechat.twitch.tv/rechat-messages?video_id=v235151000&start=1520175636.

freaktechnik/twitch-chatlog#56