By David Randall NEW YORK (Reuters) – Tesla Inc’s steep sell-off is proving to be an ongoing nightmare for fund managers that have bet heavily on the Elon Musk-led electric vehicle manufacturer. Overall, 50 actively-managed U.S. equity funds have more than 5% of their assets in the company, exceeding the barrier that many portfolio managers will not cross for one equity position to diversify their exposure. Those funds dropped by an average of 42.1% last year, more than double the average 17% decline among U.S. stock funds, according to Morningstar. The $6 billion Baron Partners Retail fund, w…