Jenkinsfile: remove redundant DOCKER_EXPERIMENTAL

DOCKER_EXPERIMENTAL is already set in the head of the file.

Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>

Jenkinsfile

@@ -408,7 +408,6 @@ pipeline {
                         }
                         stage("Integration tests") {
                             environment {
-                                DOCKER_EXPERIMENTAL = '1'
                                 DOCKER_ROOTLESS = '1'
                                 TEST_SKIP_INTEGRATION_CLI = '1'
                             }