Remove unsupported PassManager for scheduling stage

In optimization level 3's preset pass manager there was handling for a
PassManager object coming in via the `scheduling_method` argument. This
is not a valid type for the scheduling method argument and would result
in an error higher up in the call stack and also would not work for any
other optimization level. This was a leftover from Qiskit#8648 which added the
option for backends to set alternate defaults for the scheduling stage.
An earlier iteration of that PR was using a PassManager returned by the
backend instead of the plugin interface. The handling for the
PassManager input was not removed when that PR was updated to use the
plugin interface. This commit corrects the oversight and removes the
stray condition that would never work.

qiskit/transpiler/preset_passmanagers/level3.py

@@ -295,13 +295,9 @@ def _unroll_condition(property_set):
         else:
             pre_optimization = common.generate_pre_op_passmanager(remove_reset_in_zero=True)
 
-    if isinstance(scheduling_method, PassManager):
-        sched = scheduling_method
-
-    else:
-        sched = plugin_manager.get_passmanager_stage(
-            "scheduling", scheduling_method, pass_manager_config, optimization_level=3
-        )
+    sched = plugin_manager.get_passmanager_stage(
+        "scheduling", scheduling_method, pass_manager_config, optimization_level=3
+    )
 
     return StagedPassManager(
         init=init,