Do not create private filesystem namespace for /tmp

When node_exporter service is configured to use private /tmp, it can
no longer access mount points under /tmp either - node_exporter will
log errors, bump node_filesystem_device_error metric and skip metrics
related to that mount point.

templates/node_exporter.service.j2

@@ -33,7 +33,6 @@ Restart=always
 RestartSec=1
 StartLimitInterval=0
 
-PrivateTmp=yes
 {% for m in ansible_mounts if m.mount == '/home' %}
 ProtectHome=read-only
 {% else %}