qapi: Simplify QAPISchemaModularCVisitor

Since the previous commit, QAPISchemaVisitor.visit_module() is called
just once.  Simplify QAPISchemaModularCVisitor accordingly.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20191120182551.23795-7-armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
diff --git a/scripts/qapi/types.py b/scripts/qapi/types.py
index d8751da..99dcaf7 100644
--- a/scripts/qapi/types.py
+++ b/scripts/qapi/types.py
@@ -243,8 +243,9 @@
     def __init__(self, prefix):
         QAPISchemaModularCVisitor.__init__(
             self, prefix, 'qapi-types', ' * Schema-defined QAPI types',
-            __doc__)
-        self._add_system_module(None, ' * Built-in QAPI types')
+            ' * Built-in QAPI types', __doc__)
+
+    def _begin_system_module(self, name):
         self._genc.preamble_add(mcgen('''
 #include "qemu/osdep.h"
 #include "qapi/dealloc-visitor.h"