fix: revert unrelated ApiTracerFactory changes

Author: blakeli0

gax-java/gax/src/main/java/com/google/api/gax/tracing/ApiTracerFactory.java

@@ -60,9 +60,7 @@ enum OperationType {
    * @param spanName the name of the new span
    * @param operationType the type of operation that the tracer will trace
    */
-  default ApiTracer newTracer(ApiTracer parent, SpanName spanName, OperationType operationType) {
-    return BaseApiTracer.getInstance();
-  }
+  ApiTracer newTracer(ApiTracer parent, SpanName spanName, OperationType operationType);
 
   /**
    * Create a new {@link ApiTracer} that will be a child of the current context.