rpc/blockchain.cpp: Remove g_utxosetscan mutex that is only protecting a single atomic variable

src/rpc/blockchain.cpp

@@ -1976,7 +1976,6 @@ bool FindScriptPubKey(std::atomic<int>& scan_progress, const std::atomic<bool>&
 }
 
 /** RAII object to prevent concurrency issue when scanning the txout set */
-static std::mutex g_utxosetscan;
 static std::atomic<int> g_scan_progress;
 static std::atomic<bool> g_scan_in_progress;
 static std::atomic<bool> g_should_abort_scan;
@@ -1989,18 +1988,15 @@ public:
 
     bool reserve() {
         CHECK_NONFATAL(!m_could_reserve);
-        std::lock_guard<std::mutex> lock(g_utxosetscan);
-        if (g_scan_in_progress) {
+        if (g_scan_in_progress.exchange(true)) {
             return false;
         }
-        g_scan_in_progress = true;
         m_could_reserve = true;
         return true;
     }
 
     ~CoinsViewScanReserver() {
         if (m_could_reserve) {
-            std::lock_guard<std::mutex> lock(g_utxosetscan);
             g_scan_in_progress = false;
         }
     }