Remove superfluous NULL check

The curl slist API properly handle NULLs so we can be less verbose
and skip the check before passing to the slist cleanup function.
Reviewed-by: NFrancisco Guerrero <aguerrero@pivotal.io>

gpcontrib/pxf/src/libchurl.c

@@ -296,9 +296,7 @@ churl_headers_cleanup(CHURL_HEADERS headers)
 	if (!settings)
 		return;
 
-	if (settings->headers)
-		curl_slist_free_all(settings->headers);
-
+	curl_slist_free_all(settings->headers);
 	pfree(settings);
 }