qapi: Clean up superfluous null check in qapi_dealloc_type_str()

Argument can't be null.  No other Visitor method type_str() checks for
null.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
