Kel Mitchell as a young teen, when he and his partner in crime, Kenan Thompson, were introduced to the public on Nickelodeon’s “All That.”The hit show was sketch comedy, a theme song by TLC and performances from top hip-hop artists. And it was a dream for Kel, who grew up on the South Side of Chicago rapping and doing graffiti.“These are people that just a year ago I was just listening to on a CTA bus,” he told me on this week’s “Renaissance Man” of “All That’s” musical guests. “And now I’m chillin [with them].