By Pamela Barbaglia LONDON (Reuters) -CVC Capital Partners co-Chairman Steve Koltes will step down this year as Europe’s biggest buyout firm prepares for a stock market listing that a source familiar with the matter told Reuters could value it at more than 20 billion euros ($22 billion). Koltes, who co-founded CVC in 1993 as a spinout from Citibank, will leave in October “to focus on his private interests,” the firm said in a statement. He will remain on CVC’s board in a non-executive capacity. CVC recently lined up banks to work on an initial public offering (IPO), following in the footsteps …