Walt Disney Speedway (humorously referred to as the "Mickyard") was built in 1995 by the same people who own the Indianapolis Motor Speedway. It hosted the Indy 200 for five years running and a couple of NASCAR Craftsman Truck Series races in the same period, but it hasn't hosted a major professional race since 2000. Which could be why Disney is shutting it down.
Just what the theme park's plans are for the complex, we don't know. The only official word is that it's part of a plan to implement "transportation improvements" for the park using the land the speedway sits on, suggesting they may be paving (not quite) paradise to put up another parking lot.
Since no one actually races there anymore, the decision to tear down (or at least shut down) the speedway shouldn't ruffle too many of Donald's feathers. The only operation that'll be effected by the decision is the Richard Petty Experience, which offers stock car ride-alongs and supercar lapping sessions around the tri-oval track. The program is also offered in Las Vegas, Charlotte and Daytona, so it's not like it'll be shut down altogether, and could shift its supercar experience to a road course instead.