Remove unused variable (#18)

Thanks @YannRobert

Author: jdubois

src/main/java/dev/ai4j/openai4j/DefaultOpenAiClient.java

@@ -33,7 +33,6 @@ public class DefaultOpenAiClient extends OpenAiClient {
  private final OkHttpClient okHttpClient;
  private final OpenAiApi openAiApi;
  private final boolean logStreamingResponses;
- private final String userAgent;
 
  public DefaultOpenAiClient(String apiKey) {
  this(new Builder().openAiApiKey(apiKey));
@@ -42,7 +41,6 @@ public DefaultOpenAiClient(String apiKey) {
  private DefaultOpenAiClient(Builder serviceBuilder) {
  this.baseUrl = serviceBuilder.baseUrl;
  this.apiVersion = serviceBuilder.apiVersion;
- this.userAgent = serviceBuilder.userAgent;
 
  OkHttpClient.Builder okHttpClientBuilder = new OkHttpClient.Builder()
  .callTimeout(serviceBuilder.callTimeout)