qapi: Drop unused QAPIDoc member optional

Unused since commit aa964b7f "qapi2texi: Convert to QAPISchemaVisitor"
Signed-off-by: NMarkus Armbruster <armbru@redhat.com>
Message-Id: <1490015515-25851-4-git-send-email-armbru@redhat.com>
Reviewed-by: NMarc-André Lureau <marcandre.lureau@redhat.com>

scripts/qapi.py

@@ -106,7 +106,6 @@ class QAPIDoc(object):
             self.name = name
             # the list of lines for this section
             self.content = []
-            self.optional = False
 
         def append(self, line):
             self.content.append(line)