(Reuters) – Former Manchester United forward Wayne Rooney has stepped down as manager of Derby County, the League One club said on Friday, ending a turbulent 17-month spell during which his side were marred by financial trouble and relegated to the third tier. Derby were handed two separate deductions worth 21 points for entering administration and breaching financial rules last season, which eventually led to their demotion from the Championship. “Wayne Rooney has today informed Derby County Football Club that he wishes to be relieved of his duties as first team manager with immediate effect,…