README: remove bittrex (fetch support)

NOTE: hledger-flow is still available for old accounts.

README.md

@@ -155,7 +155,6 @@ However, for accounts that require fetch/import functionality, only the followin
 
 #### Available but requires community maintenance
 
-- [X] [Bittrex](https://bittrex.com/) *(non-US)*
 - [X] [Changelly](https://changelly.com/)
 - [X] [Kraken](https://kraken.com/) *(non-WA)*
 - [X] [Lofty.ai](https://lofty.ai/)
@@ -163,6 +162,7 @@ However, for accounts that require fetch/import functionality, only the followin
 
 #### Available but no longer maintained
 
+- [X] Bittrex *(bankrupt [US] / wind down [Global])*
 - [X] BlockFi *(bankrupt)*
 - [X] Celsius Network *(bankrupt)*
 - [X] Coinbase Pro *("sunsetted")*