MINOR: task: replace global_tasks_mask with a check for tree's emptiness

MINOR: task: replace global_tasks_mask with a check for tree's emptiness

This bit field used to be a per-thread cache of the result of the last
lookup of the presence of a task for each thread in the shared cache.
Since we now know that each thread has its own shared cache, a test of
emptiness is now sufficient to decide whether or not the shared tree
has a task for the current thread. Let's just remove this mask.