Remove duplicated target root check for fork choice attestation (#8277)

* Rm redundant target root check for fork choice attestation

* Comments

* Revert back verifyBeaconBlock

Co-authored-by: Raul Jordan <raul@prysmaticlabs.com>

beacon-chain/blockchain/process_attestation.go

@@ -52,10 +52,9 @@ func (s *Service) onAttestation(ctx context.Context, a *ethpb.Attestation) ([]ui
 	}
 	tgt := stateTrie.CopyCheckpoint(a.Data.Target)
 
-	// Verify beacon node has seen the target block before.
-	if !s.hasBlock(ctx, bytesutil.ToBytes32(tgt.Root)) {
-		return nil, ErrTargetRootNotInDB
-	}
+	// Note that target root check is ignored here because it was performed in sync's validation pipeline:
+	// validate_aggregate_proof.go and validate_beacon_attestation.go
+	// If missing target root were to fail in this method, it would have just failed in `getAttPreState`.
 
 	// Retrieve attestation's data beacon block pre state. Advance pre state to latest epoch if necessary and
 	// save it to the cache.

beacon-chain/blockchain/process_attestation_test.go

@@ -76,11 +76,6 @@ func TestStore_OnAttestation_ErrorConditions(t *testing.T) {
 			a:         testutil.HydrateAttestation(&ethpb.Attestation{Data: &ethpb.AttestationData{Slot: params.BeaconConfig().SlotsPerEpoch, Target: &ethpb.Checkpoint{Root: make([]byte, 32)}}}),
 			wantedErr: "slot 32 does not match target epoch 0",
 		},
-		{
-			name:      "attestation's target root not in db",
-			a:         testutil.HydrateAttestation(&ethpb.Attestation{Data: &ethpb.AttestationData{Target: &ethpb.Checkpoint{Root: bytesutil.PadTo([]byte{'A'}, 32)}}}),
-			wantedErr: "target root does not exist in db",
-		},
 		{
 			name:      "no pre state for attestations's target block",
 			a:         testutil.HydrateAttestation(&ethpb.Attestation{Data: &ethpb.AttestationData{Target: &ethpb.Checkpoint{Root: BlkWithOutStateRoot[:]}}}),