Remove Apache Maven repository from build.gradle

Samza's build.gradle still has:

    // Required for Kafka.
    maven {
      url 'https://repository.apache.org/content/groups/public'
    }

However, Kafka is now in Maven central, so this isn't needed. Removing this repository should make things faster and more stable, as Apache's Maven repo sometimes goes down, and Gradle occasionally checks it for JARs (e.g. JUnit) when it shouldn't.