Remove curlopt safe upload which is not needed since php5.6

src/Monolog/Handler/SlackWebhookHandler.php

@@ -104,9 +104,6 @@ class SlackWebhookHandler extends AbstractProcessingHandler
             CURLOPT_HTTPHEADER => ['Content-type: application/json'],
             CURLOPT_POSTFIELDS => $postString,
         ];
-        if (defined('CURLOPT_SAFE_UPLOAD')) {
-            $options[CURLOPT_SAFE_UPLOAD] = true;
-        }
 
         curl_setopt_array($ch, $options);