So what was all that "repent" and "go and sin no more" sayings Jesus said all about then?
Jesus demanded repentance, and after we are saved repentance needs to be a way of life... eschewing the things of this world in favor of the world to come. (walk in accordance with the Spirit and not the flesh)
While avoiding sin or the appearance of it won't save us, we do these things to the best of our ability in love and obedience to God..
Hebrews 10:16-17 is quoting the New Covenant promise found in Jeremiah 31: “‘This is the covenant that I will make with them after those days, declares the Lord: I will put my laws on their hearts, and write them on their minds,’ then he adds, ‘I will remember their sins and their lawless deeds no more.’”
Now, the point here is that Christ has purchased the new covenant promise, and it includes the forgiveness of our sins and the replacement of an old, unbelieving, rebellious heart with a new heart of faith and obedience.
So yes, sin should be spoken of in our churches, and Christ still speaks to us of sin today, when he changes our rebellious nature to one where we seek to follow God's will in our lives.