Drop typing of self.idle

As `self.idle` is actually a `SortedSet` instead of a `set`, we can't
type it. So revert changes related to `self.idle`. Though leave all
other `set`-based typing for other optimizations to be applied where
possible.

distributed/scheduler.py

@@ -4958,13 +4958,12 @@ def check_idle_saturated(self, ws, occ: double = -1.0):
         total_occupancy: double = self.total_occupancy
         avg: double = total_occupancy / total_nthreads
 
-        idle: set = self.idle
         saturated: set = self.saturated
         if p < nc or occ / nc < avg / 2:
-            idle.add(ws)
+            self.idle.add(ws)
             saturated.discard(ws)
         else:
-            idle.discard(ws)
+            self.idle.discard(ws)
 
             pending: double = occ * (p - nc) / p / nc
             if p > nc and pending > 0.4 and pending > 1.9 * avg: