I'm assuming that by the word "requirement" you're speaking of the requirements for salvation specifically, and not for being a member of a church, or staying in good standing at work or with friends.
Short version: Circumcision is no longer required because Christ has already fulfilled the requirement.
Longer version: To be sure, the entirety of the law no longer applies to the Christian, on account of the fact that every requirement of the law has already been met within Christ. That means that the penalty that a Christian might owe for failing to be circumcised has already been paid for by Christ, who I would have to insist, though it doesn't directly say in Scripture, was circumcised.
That also means that it's not a requirement for Christians to "not murder", "not steal", "not lie", "not commit adultery", etc. etc. etc. While it's still a heinous thing to murder, steal, lie or commit adultery, it is no longer a requirement for a Christian's salvation to be free of these things. The penalty for all of them is paid for by Christ. Thus, for my salvation, I am no longer required to have avoided murder, adultery, stealing for my entire life in order for me to be righteous in God's eyes, because my righteousness no longer comes from myself, but from Christ's sacrifice instead.
So in other words, while circumcision is still "required" so to speak, if you were to try to attain salvation by your works, since we no longer seek to be justified by works, circumcision is no longer required.
In other words, through God's grace in giving us Christ, we are now freed from the requirements of the law. All of them.
It's also important to keep in mind what Paul says about living by circumcision in Galatians 5:1-6:
For freedom Christ has set us free; stand firm therefore, and do not submit again to a yoke of slavery.
Look: I, Paul, say to you that if you accept circumcision, Christ will
be of no advantage to you. I testify again to every man who accepts
circumcision that he is obligated to keep the whole law. You are
severed from Christ, you who would be justified by the law; you have
fallen away from grace. For through the Spirit, by faith, we ourselves
eagerly wait for the hope of righteousness. For in Christ Jesus
neither circumcision nor uncircumcision counts for anything, but only
faith working through love.
We have been freed from the law by Christ. If we accept circumcision as a requirement for salvation, then we are no longer living by the grace of Christ, but we are living by the law. And if we're living by the law, then we are required to live by the WHOLE weight of the law, not just circumcision.
Therefore, since the penalty has been paid, and the judgement has been rendered, we are free, and no longer required to be circumcised.