contint: rm /var/lib/jenkins/tmpfs no more needed

/var/lib/jenkins/tmpfs is a 512M tmpfs to speed up tests, it was used
when we ran jobs in the Jenkins master instance.  Nowadays, we run all
tests in slave instances which use /var/lib/jenkins-slave/tmpfs

Change-Id: Ia92b940dfc65d4358b08a755449089bcaaa22922