test: remove chain backdating from integration tests

fixes #12557

itests/kit/ensemble.go

@@ -1089,7 +1089,7 @@ func (n *Ensemble) generateGenesis() *genesis.Template {
 		Accounts:         n.genesis.accounts,
 		Miners:           n.genesis.miners,
 		NetworkName:      "test",
-		Timestamp:        uint64(time.Now().Unix() - int64(n.options.pastOffset.Seconds())),
+		Timestamp:        uint64(time.Now().Unix()),
 		VerifregRootKey:  verifRoot,
 		RemainderAccount: gen.DefaultRemainderAccountActor,
 	}

itests/kit/ensemble_opts.go

@@ -1,8 +1,6 @@
 package kit
 
 import (
-	"time"
-
 	"github.com/filecoin-project/go-state-types/abi"
 
 	"github.com/filecoin-project/lotus/build"
@@ -19,7 +17,6 @@ type genesisAccount struct {
 }
 
 type ensembleOpts struct {
-	pastOffset   time.Duration
 	verifiedRoot genesisAccount
 	accounts     []genesisAccount
 	mockProofs   bool
@@ -28,7 +25,6 @@ type ensembleOpts struct {
 }
 
 var DefaultEnsembleOpts = ensembleOpts{
-	pastOffset: 10000000 * time.Second, // time sufficiently in the past to trigger catch-up mining.
 	upgradeSchedule: stmgr.UpgradeSchedule{{
 		Height:  -1,
 		Network: buildconstants.TestNetworkVersion,