[wallet] setup wallet background flushing in WalletInit directly

WalletInit::Start calls postInitProcess() for each wallet. Previously
each call to postInitProcess() would attempt to schedule wallet
background flushing.

Just start wallet background flushing once from WalletInit::Start().

src/wallet/wallet.cpp

@@ -23,7 +23,6 @@
 #include <primitives/block.h>
 #include <primitives/transaction.h>
 #include <script/script.h>
-#include <scheduler.h>
 #include <timedata.h>
 #include <txmempool.h>
 #include <utilmoneystr.h>
@@ -4308,18 +4307,11 @@ CWallet* CWallet::CreateWalletFromFile(const std::string& name, const fs::path&
     return walletInstance;
 }
 
-std::atomic<bool> CWallet::fFlushScheduled(false);
-
-void CWallet::postInitProcess(CScheduler& scheduler)
+void CWallet::postInitProcess()
 {
     // Add wallet transactions that aren't already in a block to mempool
     // Do this here as mempool requires genesis block to be loaded
     ReacceptWalletTransactions();
-
-    // Run a thread to flush wallet periodically
-    if (!CWallet::fFlushScheduled.exchange(true)) {
-        scheduler.scheduleEvery(MaybeCompactWalletDB, 500);
-    }
 }
 
 bool CWallet::BackupWallet(const std::string& strDest)