'I came not for x but for y' doesn't exclude the performing of x, it only means that isn't the chief purpose (this is true even when x always accompanies y), or in this case chief calling or mission.
This is more or less proven to be the contextual meaning by the next clause: "not in wisdom of word, [but in simplicity,] lest the gospel of Christ be brought to nothing." This obviously doesn't mean St. Paul says nothing wise, but that to seek to explain the gospel in full depth, or even with the 'wisdom of man,' sophistry, would deter people from grasping its fundamental force and meaning and in fact stand a hindrance to the gospel than an incentive and aid to its acceptance.
But more importantly, the previous verses explicity tells us St. Paul baptized people:
1 Corinthians 1:13-16 (DRB) Is Christ divided? Was Paul then crucified for you? or were you baptized in the name of Paul? I give God thanks, that I baptized none of you but Crispus and Caius; Lest any should say that you were baptized in my name. And I baptized also the household of Stephanus; besides, I know not whether I baptized any other.
Clearly, he was intimately involved with baptizing plenty of people in the community. Paul himself was first 'unleashed' to his new mission in being baptized by one sent by Jesus to baptized Him into the Body--to officially "put on Christ" (Gal 3:27) and shake off the old Saul. It just wasn't Paul's mission specifically to baptize; and that he didn't come to baptize but to preach isn't even his main point, but a passing one, in a somewhat angry reaction to their misplaced identification with certain pastors of the Church rather than its Head (creating factions within the local church). "Where you baptized in the name of Paul?" is a daring way of reminding His readers that they should be focused on that Name into which they were really baptized: "the Name of the Father, the of Son, and of the Holy Ghost" (Mt 28:19). His mission was one principally of spreading the gospel throughout the world, in the form of his personal teaching, and those he ordained to the same ministry of spreading the same message and doctrine, but especially in his Epistles which are now codified in Sacred Scripture.
This follows the example of Christ, who had their same mission (Jn 20:21), who also had others perform baptisms (Jn 4:2), while He focused on preaching the gospel, when it was 'in its infancy' so to speak (its foundations being laid).