chore: remove ancient setting migration logic

This code in its original form was first introduced in 0.12 (bbbbdd8)
and in its current form 0.17 (ae506ba) and has survived rebrandings.
Time to retire it.

src/qt/optionsmodel.cpp

@@ -232,13 +232,8 @@ void OptionsModel::Init(bool resetSettings)
     if (!gArgs.SoftSetArg("-coinjoinrounds", settings.value("nCoinJoinRounds").toString().toStdString()))
         addOverriddenOption("-coinjoinrounds");
 
-    if (!settings.contains("nCoinJoinAmount")) {
-        // for migration from old settings
-        if (!settings.contains("nAnonymizeDashAmount"))
-            settings.setValue("nCoinJoinAmount", DEFAULT_COINJOIN_AMOUNT);
-        else
-            settings.setValue("nCoinJoinAmount", settings.value("nAnonymizeDashAmount").toInt());
-    }
+    if (!settings.contains("nCoinJoinAmount"))
+        settings.setValue("nCoinJoinAmount", DEFAULT_COINJOIN_AMOUNT);
     if (!gArgs.SoftSetArg("-coinjoinamount", settings.value("nCoinJoinAmount").toString().toStdString()))
         addOverriddenOption("-coinjoinamount");