Remove outdated error for used_defined_tests

Previously, a user-defined test (those run with "Test" instead of "Run") would cause a flash message and a Sentry error if anything was printed to StdErr. As this might happen during user code execution (and therefore is no error), we remove this warning. All existing errors are already caught (compare with enum status in testrun.rb), so it's fine for now.

Fixes CODEOCEAN-BT

app/assets/javascripts/editor/editor.js.erb

@@ -705,12 +705,6 @@ var CodeOceanEditor = {
             this.showContainerDepletedMessage();
         } else if (output.status === 'out_of_memory') {
             this.showOutOfMemoryMessage();
-        } else if (output.stderr) {
-            $.flash.danger({
-                icon: ['fa-solid', 'fa-bug'],
-                text: $('#run').data('message-failure')
-            });
-            Sentry.captureException(JSON.stringify(output));
         }
     },