Starbucks has suddenly replaced its chief executive after the company suffered a bigger-than-expected drop in sales.The coffee chain's board announced on Tuesday that Laxman Narasimhan had departed the firm "effective immediately". They said Brian Niccol - the boss of burrito chain Chipotle - would soon take his place.It comes following mounting pressure and speculation over the future of Mr Narasimhan, who had only been in the role for a year-and-a-half.Last month, the firm reported that global sales had f ...