Since the previous commit, QAPISchemaVisitor.visit_module() is called
just once.  Simplify QAPISchemaModularCVisitor accordingly.
Signed-off-by: NMarkus Armbruster <armbru@redhat.com>
Message-Id: <20191120182551.23795-7-armbru@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>