chore: Remove unused code

Author: lqiu96

google-cloud-bigquerystorage/src/main/java/com/google/cloud/bigquery/storage/v1/JsonToProtoMessage.java

@@ -131,8 +131,6 @@ public class JsonToProtoMessage implements ToProtoConverter<Object> {
   // or end of string
   private static final Pattern ISO8601_TIMESTAMP_HIGH_PRECISION_PATTERN =
       Pattern.compile("\\.(\\d{10,})(?:\\D|$)");
-  private static final long MICROS_PER_SECOND = 1_000_000;
-  private static final int NANOS_PER_MICRO = 1_000;
 
   /** You can use {@link #INSTANCE} instead */
   public JsonToProtoMessage() {}