python: Use more standard shared dbscheme sections

We now use the shared "Overlay support" and "Database metadata".

python/extractor/semmle/dbscheme.template

@@ -40,20 +40,26 @@ externalData(
   string value : string ref
 );
 
+/*- Database metadata -*/
+
+/**
+ * The CLI will automatically emit applicable tuples for this table,
+ * such as `databaseMetadata("isOverlay", "true")` when building an
+ * overlay database.
+ */
+databaseMetadata(
+  string metadataKey: string ref,
+  string value: string ref
+);
+
 /*- Overlay support -*/
 
 /**
- * The CLI will automatically emit the tuple `databaseMetadata("isOverlay", "true")`,
- * along with an `overlayChangedFiles` tuple for each new/modified/deleted file,
- * when building an overlay database, and these can be used by the discard predicates.
+ * The CLI will automatically emit tuples for each new/modified/deleted file
+ * when building an overlay database.
  */
-databaseMetadata(
-  string metadataKey : string ref,
-  string value : string ref
-);
-
 overlayChangedFiles(
-  string path : string ref
+  string path: string ref
 );
 
 /*- DEPRECATED: Snapshot date -*/