Well, yes and no. The Eucharist actually does wipe away venial sin, without sacramental confession. As for mortal sin, to receive Holy Communion with an unconfessed mortal sin only compounds the offense. But no, that does not mean that the Eucharist is meant to replace sacramental confession in terms of being a source of grace and a help in the formation and tone of our consciences, even when we have not committed a mortal sin:
From the Catechism:
1393 Holy Communion separates us from sin. The body of Christ we receive in Holy Communion is “given up for us,” and the blood we drink “shed for the many for the forgiveness of sins.” For this reason the Eucharist cannot unite us to Christ without at the same time cleansing us from past sins and preserving us from future sins:
For as often as we eat this bread and drink the cup, we proclaim the death of the Lord. If we proclaim the Lord’s death, we proclaim the forgiveness of sins. If, as often as his blood is poured out, it is poured for the forgiveness of sins, I should always receive it, so that it may always forgive my sins. Because I always sin, I should always have a remedy.230
1395 By the same charity that it enkindles in us, the Eucharist preserves us from future mortal sins. The more we share the life of Christ and progress in his friendship, the more difficult it is to break away from him by mortal sin. The Eucharist is not ordered to the forgiveness of mortal sins - that is proper to the sacrament of Reconciliation. The Eucharist is properly the sacrament of those who are in full communion with the Church.
1496 The spiritual effects of the sacrament of Penance are:
- reconciliation with God by which the penitent recovers grace;
- reconciliation with the Church;
- remission of the eternal punishment incurred by mortal sins;
- remission, at least in part, of temporal punishments resulting from sin;
- peace and serenity of conscience, and spiritual consolation;
- an increase of spiritual strength for the Christian battle.
In document after document on the subject, the Church teaches that the Sacrament of Penance works hand-in-glove with the Holy Eucharist, although both wipe away venial sin. Confession helps us particularly both to have peace of conscience and to avoid sin in the future. We are not meant to depend entirely on the Eucharist for this, you are right, but to take our venial sins to confession in order to obtain grace from both sacraments, towards that end.
So for instance, if we do not confess a venial sin that has been wiped away by reception of the Eucharist, the sin is still gone. We have missed an opportunity to do better in battling the sin in the future, though, and also have given in to what can become the very bad habit of not mentioning a sin simply because we aren’t required to confess it. If a sin smites our conscience, we should still confess it in sacramental confession, even if the Holy Eucharist will have wiped it away, because we will benefit so much by having admitted it and renounced it out loud within the sacrament of healing.