remove is_started method

Signed-off-by: Shijie Sheng <liouvetren@gmail.com>

Author: shijiesheng

cadence/_internal/workflow/workflow_intance.py

@@ -23,9 +23,6 @@ def start(self, input: Payload):
             workflow_input = self._data_converter.from_data(input, [Any])
             self._task = self._loop.create_task(run_method(*workflow_input))
 
-    def is_started(self) -> bool:
-        return self._task is not None
-
     def run_once(self):
         self._loop.run_until_yield()