fix: remove .max_characters from ElementMetadata (#2032)

This metadata field is assumedly vestigial and is unused by any code in
the repo. `max_characters` is an optional argument to `chunk_by_title()`
and has meaning in that context, but is not written to the metadata.

Remove this unused field.

test_unstructured/staging/test_base_staging.py

@@ -157,7 +157,6 @@ def test_default_pandas_dtypes():
             emphasized_text_tags=["emphasized", "text", "tags"],
             text_as_html="text_as_html",
             regex_metadata={"key": [RegexMetadata(text="text", start=0, end=4)]},
-            max_characters=2,
             is_continuation=True,
             detection_class_prob=0.5,
         ),

unstructured/documents/elements.py

@@ -188,7 +188,6 @@ class ElementMetadata:
     regex_metadata: Optional[Dict[str, List[RegexMetadata]]] = None
 
     # Chunking metadata fields
-    max_characters: Optional[int] = None
     is_continuation: Optional[bool] = None
 
     # Detection Model Class Probabilities from Unstructured-Inference Hi-Res