feat(core): drop support for after-step-hook

This was necessary for hooking display.refresh() with the old UI toolkit.
With the new one, we explicitly refresh the display after every paint, so
implicit after-step refresh seems no longer necessary.

core/src/trezor/loop.py

@@ -20,9 +20,6 @@
     AwaitableTask = Task | Awaitable
     Finalizer = Callable[[Task, Any], None]
 
-# function to call after every task step
-after_step_hook: Callable[[], None] | None = None
-
 # tasks scheduled for execution in the future
 _queue = utimeq.utimeq(64)
 
@@ -179,8 +176,6 @@ def _step(task: Task, value: Any) -> None:
         else:
             if __debug__:
                 log.error(__name__, "unknown syscall: %s", result)
-        if after_step_hook:
-            after_step_hook()
 
 
 class Syscall:

core/src/trezor/ui/__init__.py

@@ -51,12 +51,6 @@ def refresh() -> None:
 else:
     refresh = display.refresh  # type: ignore [obscured-by-same-name]
 
-
-# in both debug and production, emulator needs to draw the screen explicitly
-if utils.EMULATOR or utils.INTERNAL_MODEL in ("T1B1", "T2B1"):
-    loop.after_step_hook = refresh
-
-
 # import style later to avoid circular dep
 from trezor.ui import style  # isort:skip