Remove closed exchanges.

[Zapata]

Following end of 2017 China ban of crypto exchanges, many exchange have closed.

At least those sources are impacted:

• BTC38 (API/website down),
• BTCChina (API/website down),
• Yunbi (API/Website down)
  • BTer (API/Website down)
• ChBTC? (API still respond, but data seems old)

[Zapata]

You can find in my repository a cleaned list of sources.

You can cherry pick those commits: BTC38, BtsChina, BTer, Yunbi