Fix handling subsection tags in input files (#1371)

Another tiny fix in the code which process documentation. This handles the `<subsection></subsection>` tags, like in Mathics-Django documentation.

Author: mmatera

mathics/doc/structure.py

@@ -554,15 +554,7 @@ def load_part_from_file(
                         chapter, title, text, operator=None, installed=True
                     )
                     chapter.sections.append(section)
-                    subsections = SUBSECTION_RE.findall(text)
-                    for subsection_title in subsections:
-                        subsection = self.subsection_class(
-                            chapter,
-                            section,
-                            subsection_title,
-                            text,
-                        )
-                        section.subsections.append(subsection)
+                    # Subsections are processed inside the Documentation entry.
                 else:
                     section = None
                 if not chapter.doc: