Here is another option: delete the builds remotely with cURL. (Replace the beginning of the URLs with whatever you use to access Jenkins with your browser.)
$ curl -X POST http://jenkins-host.tld:8080/jenkins/job/myJob/[1-56]/doDeleteAll
The above deletes build #1 to #56 for job myJob.
If authentication is enabled on the Jenkins instance, a user name and API token must be provided like this:
$ curl -u userName:apiToken -X POST http://jenkins-host.tld:8080/jenkins/job/myJob/[1-56]/doDeleteAll
The API token must be fetched from the /me/configure page in Jenkins. Just click on the “Show API Token…” button to display both the user name and the API token.
Edit: As pointed out by yegeniy in a comment below, one might have to replace doDeleteAll by doDelete in the URLs above to make this work, depending on the configuration.