fix: Remove orphaned _credential initializer from PollingDataSource constructor

The _credential field was removed but the constructor initializer list
still referenced it, causing a compilation error.

Co-Authored-By: [email protected] <[email protected]>

Author: devin-ai-integration[bot]

packages/common_client/lib/src/data_sources/polling_data_source.dart

@@ -68,8 +68,7 @@ final class PollingDataSource implements DataSource {
           _defaultClientFactory})
       : _endpoints = endpoints,
         _logger = logger.subLogger('PollingDataSource'),
-        _dataSourceConfig = dataSourceConfig,
-        _credential = credential {
+        _dataSourceConfig = dataSourceConfig {
     _pollingInterval = testingInterval ?? dataSourceConfig.pollingInterval;
 
     if (_dataSourceConfig.useReport) {