consensus: Remove redundant contextual consensus rules

By the time we reach any of these rules, we have already rejected
transactions without fOverwintered set, so we can assume that it is set.

We replace the vague "overwinter is active" message with a more specific
"overwintered flag must be set" from one of the removed rules.

src/main.cpp

@@ -813,8 +813,8 @@ bool ContextualCheckTransaction(
         if (!tx.fOverwintered) {
             return state.DoS(
                 dosLevelConstricting,
-                error("ContextualCheckTransaction: overwinter is active"),
-                REJECT_INVALID, "tx-overwinter-active");
+                error("ContextualCheckTransaction: fOverwintered flag must be set when Overwinter is active"),
+                REJECT_INVALID, "tx-overwintered-flag-not-set");
         }
 
         // Check that all transactions are unexpired
@@ -830,23 +830,15 @@ bool ContextualCheckTransaction(
         // Rules that became inactive after Sapling activation.
         if (!saplingActive) {
             // Reject transactions with invalid version
-            if (tx.fOverwintered && tx.nVersion > OVERWINTER_MAX_TX_VERSION ) {
+            if (tx.nVersion > OVERWINTER_MAX_TX_VERSION ) {
                 return state.DoS(
                     dosLevelPotentiallyRelaxing,
                     error("CheckTransaction(): overwinter version too high"),
                     REJECT_INVALID, "bad-tx-overwinter-version-too-high");
             }
 
-            // Reject transactions with valid version but missing overwinter flag
-            if (tx.nVersion >= OVERWINTER_MIN_TX_VERSION && !tx.fOverwintered) {
-                return state.DoS(
-                    dosLevelConstricting,
-                    error("ContextualCheckTransaction(): overwinter flag must be set"),
-                    REJECT_INVALID, "tx-overwinter-flag-not-set");
-            }
-
             // Reject transactions with non-Overwinter version group ID
-            if (tx.fOverwintered && tx.nVersionGroupId != OVERWINTER_VERSION_GROUP_ID) {
+            if (tx.nVersionGroupId != OVERWINTER_VERSION_GROUP_ID) {
                 return state.DoS(
                     dosLevelPotentiallyRelaxing,
                     error("CheckTransaction(): invalid Overwinter tx version"),
@@ -857,32 +849,24 @@ bool ContextualCheckTransaction(
 
     // Rules that apply to Sapling and later:
     if (saplingActive) {
-        // Reject transactions with valid version but missing overwintered flag
-        if (tx.nVersion >= SAPLING_MIN_TX_VERSION && !tx.fOverwintered) {
-            return state.DoS(
-                dosLevelConstricting,
-                error("ContextualCheckTransaction(): overwintered flag must be set"),
-                REJECT_INVALID, "tx-overwintered-flag-not-set");
-        }
-
         // Reject transactions with non-Sapling version group ID
-        if (tx.fOverwintered && tx.nVersionGroupId != SAPLING_VERSION_GROUP_ID) {
+        if (tx.nVersionGroupId != SAPLING_VERSION_GROUP_ID) {
             return state.DoS(
                 dosLevelPotentiallyRelaxing,
                 error("CheckTransaction(): invalid Sapling tx version"),
                 REJECT_INVALID, "bad-sapling-tx-version-group-id");
         }
 
         // Reject transactions with invalid version
-        if (tx.fOverwintered && tx.nVersion < SAPLING_MIN_TX_VERSION ) {
+        if (tx.nVersion < SAPLING_MIN_TX_VERSION) {
             return state.DoS(
                 dosLevelConstricting,
                 error("CheckTransaction(): Sapling version too low"),
                 REJECT_INVALID, "bad-tx-sapling-version-too-low");
         }
 
         // Reject transactions with invalid version
-        if (tx.fOverwintered && tx.nVersion > SAPLING_MAX_TX_VERSION ) {
+        if (tx.nVersion > SAPLING_MAX_TX_VERSION) {
             return state.DoS(
                 dosLevelPotentiallyRelaxing,
                 error("CheckTransaction(): Sapling version too high"),