[Doc] Remove ThreadFlushWalletDB from developer notes

It was removed in #2150

Author: random-zebra

doc/developer-notes.md

@@ -301,8 +301,6 @@ Threads
 
 - DumpAddresses : Dumps IP addresses of nodes to peers.dat.
 
-- ThreadFlushWalletDB : Close the wallet.dat file if it hasn't been used in 500ms.
-
 - ThreadRPCServer : Remote procedure call handler, listens on port 8332 for connections and services them.
 
 - BitcoinMiner : Generates PIVs (if wallet is enabled).