fix: remove 500 char path name limit (googleapis#1865)

Remove path name 500 character limit validation as this is impacting some customers for key retrievals.

Author: cindy-peng

google-cloud-datastore/src/main/java/com/google/cloud/datastore/PathElement.java

@@ -128,7 +128,6 @@ static PathElement of(String kind) {
 
   public static PathElement of(String kind, String name) {
     checkArgument(!Strings.isNullOrEmpty(name), "name must not be empty or null");
-    checkArgument(name.length() <= 500, "name must not exceed 500 characters");
     return new PathElement(kind, name, null);
   }