LOS ANGELES — Los Angeles County supervisors voted unanimously Tuesday to buy up and forgive millions of dollars in medical debt as part of a comprehensive plan to tackle a $2.9 billion burden that weighs on almost 800,000 residents. The measure, authored by supervisors Janice Hahn and Holly Mitchell, allows the county to enter into a pilot program with Undue Medical Debt, previously known as RIP Medical Debt, a national organization that relieves patients of what they owe by purchasing their medical debt for pennies on the dollar then retiring it. “Medical debt is largely out of people’s cont…