Remove an unused method

`TextChangelogFormat.version_title_text` never worked: it's missing a
parameter in the call to `re.sub`. Fortunately it's unused, so just remove it.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>

scripts/assemble_changelog.py

@@ -93,11 +93,6 @@ class ChangelogFormat:
         """
         raise NotImplementedError
 
-    @classmethod
-    def version_title_text(cls, version_title):
-        """Return the text of a formatted version section title."""
-        raise NotImplementedError
-
     @classmethod
     def split_categories(cls, version_body):
         """Split a changelog version section body into categories.
@@ -141,10 +136,6 @@ class TextChangelogFormat(ChangelogFormat):
                 top_version_title, top_version_body,
                 changelog_file_content[top_version_end:])
 
-    @classmethod
-    def version_title_text(cls, version_title):
-        return re.sub(r'\n.*', version_title, flags=re.DOTALL)
-
     _category_title_re = re.compile(r'(^\w.*)\n+', re.MULTILINE)
     @classmethod
     def split_categories(cls, version_body):