bpo-35715: Liberate return value of _process_worker (GH-11514)

bpo-35715: Liberate return value of _process_worker (GH-11514)

ProcessPoolExecutor workers will hold the return value of their last task in memory until the next task is received. Since the return value has already been propagated to the parent process's Future (or has been discarded by this point), the object can be safely released.