Revert 954c16c since it was unnecessary

The alledged issue didn't exist and checkpoint folders are created by
the keras callback automatically already.

I misread tests on this and didn't catch that the problem I was seeing
was caused by a different part of my pipeline.

Author: han-ol

bayesflow/workflows/basic_workflow.py

@@ -125,10 +125,6 @@ def __init__(
                 file_ext = self.checkpoint_name + ".weights.h5"
             else:
                 file_ext = self.checkpoint_name + ".keras"
-
-            # If the directory checkpoint_filepath does not exist yet, create it.
-            os.makedirs(self.checkpoint_filepath, exist_ok=True)
-
             checkpoint_full_filepath = os.path.join(self.checkpoint_filepath, file_ext)
             if os.path.exists(checkpoint_full_filepath):
                 msg = (