exchanges: Remove Changelly

Changelly.com no longer has DCR since 2023-09-19 or earlier.

On 2023-10-19 their support team replied that DCR was temporarily
switched off but as of this commit DCR is still missing from the
dropdown at changelly.com and also from their new System Health
dashboard (https://pro.changelly.com/system-monitor).

Removing for now, we can add it back if they enable DCR again.

src/data/exchanges/exchanges.yml

@@ -45,11 +45,6 @@
   tags:
   name: OKX
   highlight: false
--
-  url: "https://changelly.com/"
-  tags:
-  name: Changelly
-  highlight: false
 -
   url: "https://www.bitladon.com/"
   tags: