Remove the nullability specifier since the return signature has changed to be Void.

Author: visumickey

firebase-perf/src/main/java/com/google/firebase/perf/metrics/validator/PerfMetricValidator.java

@@ -139,7 +139,6 @@ public static String validateMetricName(@Nullable String str) {
    * @return null if the entry can be used as an Attribute, if not, an error string explaining why
    *     it can't be used.
    */
-  @Nullable
   public static void validateAttribute(@NonNull String key, @NonNull String value) {
     if (key == null || key.length() == 0) {
       throw new IllegalArgumentException("Attribute key must not be null or empty");