fix: remove unused variable in test_unspecified_reader_falls_back_silently

Pre-commit ci caught unused variable original_readers that was defined but never
used. Remove it to pass ruff check.

Signed-off-by: Cipher <cipher@openclaw.ai>

Author: Cipher

tests/transforms/test_load_image.py

@@ -532,7 +532,6 @@ def test_unspecified_reader_falls_back_silently(self):
 
         # Patch SUPPORTED_READERS entries to raise OptionalImportError
         # This simulates optional packages not being installed
-        original_readers = {}
         from monai.transforms.io.array import SUPPORTED_READERS
 
         # Patch a few readers to fail (e.g., ITKReader)