Script to remove old jobs. Go to Manage Jenkins -> script console and paste this script. Just change MAX_BUILDS for the number of jobs you want to keep

Remove old Jenkins jobs

MAX_BUILDS = 10 // max builds to keep
 
def jobs = Jenkins.instance.items;
def branchesArray = ["master", "develop"] as String[];

def deleteJobs(job) {
    def recent = job.builds.limit(MAX_BUILDS)
    println "Recent Builds: "  + recent
    println "============================="

    for (build in job.builds) {
        if (!recent.contains(build) && !build.isBuilding()) {
            println "Deleting: " + build
            build.delete()
            println ""
        }
    }
}
 
for (job in jobs) {
    println "Job: " + job.name
    try {
        if (job instanceof jenkins.branch.MultiBranchProject) {
            println "Multibranch"

            for (branch in branchesArray) {
                println "Branch " + branch
                job = job.getJob(branch)
                deleteJobs(job)
            }
        } else {
            deleteJobs(job)
        }
    } catch(Exception ex) {
        continue
    }
}