If you want to clear the build history of MultiBranchProject (e.g. pipeline), go to your Jenkins home page ? Manage Jenkins ? Script Console and run the following script:
def projectName = "ProjectName"
def project = Jenkins.instance.getItem(projectName)
def jobs = project.getItems().each {
def job = it
job.getBuilds().each { it.delete() }
job.nextBuildNumber = 1
job.save()
}