City lawyer Ian Rosenblatt has launched legal action in London’s High Court against music mogul Simon Cowell over alleged withheld fees. The lawsuit stems from the lawyer stepping down as director of Syco Entertainment, Cowell’s production company, and as an adviser to the talent show judge. The High Court filings show Quinn Emanuel, which is representing Rosenblatt, filed the claim in the Commercial Court on Wednesday. Rosenblatt served on Cowe…