Remove references to ISBN

Remove references to ISBN requests.

v1/citoid.yaml

@@ -21,11 +21,9 @@ paths:
         - Citation
       summary: Get citation data given an article identifier.
       description: |
-        Generates citation data given a URL, DOI, PMID, ISBN, or PMCID.
+        Generates citation data given a URL, DOI, PMID, or PMCID.
 
-        Because requests for ISBNs go to WorldCat and we have a limited number of these requests to their API per day, please limit requests to 100 per user or bot per day.
-
-        Automated requests for URLs and DOIs can be made at a rate of 1 request per second (rps).
+        Automated requests can be made at a rate of 1 request per second (rps).
 
         See more at [Citoid service documentation](https://www.mediawiki.org/wiki/Citoid)