fix: simplify AthenaDataset._is_valid method

yt/frontends/athena/data_structures.py

@@ -632,12 +632,7 @@ def _parse_parameter_file(self):
 
  @classmethod
  def _is_valid(cls, filename, *args, **kwargs):
- try:
- if "vtk" in filename:
- return True
- except Exception:
- pass
- return False
+ return filename.endswith(".vtk")
 
  @property
  def _skip_cache(self):