Remove schema:subjectOf edges from KG extraction (#695)

The subjectOf triples were redundant with the subgraph provenance model
introduced in e8407b34. Entity-to-source lineage can be traced via
tg:contains -> subgraph -> prov:wasDerivedFrom -> chunk, making the
direct subjectOf edges unnecessary metadata polluting the knowledge graph.

Removed from all three extractors (agent, definitions, relationships),
cleaned up the SUBJECT_OF constant and vocabulary label, and updated
tests accordingly.

trustgraph-base/trustgraph/provenance/namespaces.py

@@ -30,7 +30,6 @@ RDFS_LABEL = RDFS + "label"
 
 # Schema.org namespace
 SCHEMA = "https://schema.org/"
-SCHEMA_SUBJECT_OF = SCHEMA + "subjectOf"
 SCHEMA_DIGITAL_DOCUMENT = SCHEMA + "DigitalDocument"
 SCHEMA_DESCRIPTION = SCHEMA + "description"
 SCHEMA_KEYWORDS = SCHEMA + "keywords"

trustgraph-base/trustgraph/provenance/vocabulary.py

@@ -16,7 +16,7 @@ from . namespaces import (
     PROV_WAS_DERIVED_FROM, PROV_WAS_GENERATED_BY,
     PROV_USED, PROV_WAS_ASSOCIATED_WITH, PROV_STARTED_AT_TIME,
     DC_TITLE, DC_SOURCE, DC_DATE, DC_CREATOR,
-    SCHEMA_SUBJECT_OF, SCHEMA_DIGITAL_DOCUMENT, SCHEMA_DESCRIPTION,
+    SCHEMA_DIGITAL_DOCUMENT, SCHEMA_DESCRIPTION,
     SCHEMA_KEYWORDS, SCHEMA_NAME,
     SKOS_DEFINITION,
     TG_CONTAINS, TG_PAGE_COUNT, TG_MIME_TYPE, TG_PAGE_NUMBER,
@@ -63,7 +63,6 @@ DC_PREDICATE_LABELS = [
 
 # Schema.org labels
 SCHEMA_LABELS = [
-    _label_triple(SCHEMA_SUBJECT_OF, "subject of"),
     _label_triple(SCHEMA_DIGITAL_DOCUMENT, "Digital Document"),
     _label_triple(SCHEMA_DESCRIPTION, "description"),
     _label_triple(SCHEMA_KEYWORDS, "keywords"),

trustgraph-base/trustgraph/rdf.py

@@ -2,7 +2,6 @@
 RDF_TYPE = "http://www.w3.org/1999/02/22-rdf-syntax-ns#type"
 RDF_LABEL = "http://www.w3.org/2000/01/rdf-schema#label"
 DEFINITION = "http://www.w3.org/2004/02/skos/core#definition"
-SUBJECT_OF = "https://schema.org/subjectOf"
 
 TRUSTGRAPH_ENTITIES = "http://trustgraph.ai/e/"