Revert "trezor/loop: optimization"

This reverts commit cfa1705a88.

In MicroPython, list.clear() does not memset the backing buffer, preventing the
paused tasks from being garbage collected.

src/trezor/loop.py

@@ -53,7 +53,7 @@ def unschedule(task):
 
 
 def pause(task, iface):
-    tasks = _paused.get(iface)
+    tasks = _paused.get(iface, None)
     if tasks is None:
         tasks = _paused[iface] = []
     tasks.append(task)
@@ -94,11 +94,9 @@ def run():
 
         if io.poll(_paused, msg_entry, delay):
             # message received, run tasks paused on the interface
-            msg_tasks = _paused.get(msg_entry[0])
-            if msg_tasks is not None:
-                for task in msg_tasks:
-                    _step(task, msg_entry[1])
-                msg_tasks.clear()
+            msg_tasks = _paused.pop(msg_entry[0], ())
+            for task in msg_tasks:
+                _step(task, msg_entry[1])
         else:
             # timeout occurred, run the first scheduled task
             if _queue: