Remove zkgroup enabled flag

The last remnants of a time before zkgroup have been swept away.

service/src/main/java/org/whispersystems/textsecuregcm/configuration/ZkConfig.java

@@ -26,10 +26,6 @@ public class ZkConfig {
   @NotNull
   private byte[] serverPublic;
 
-  @JsonProperty
-  @NotNull
-  private Boolean enabled;
-
   public byte[] getServerSecret() {
     return serverSecret;
   }
@@ -37,8 +33,4 @@ public class ZkConfig {
   public byte[] getServerPublic() {
     return serverPublic;
   }
-
-  public boolean isEnabled() {
-    return enabled;
-  }
 }