mgr/dashboard: Remove hard-coded timezone off Grafana dashboards

mgr/dashboard: Remove hard-coded timezone off Grafana dashboards

Remove hard-coded timezone off Grafana dashboards to enable the Grafana
administrator to decide which timezone should be used for dashboards.

If we hard-coded those values, changing the global settings in Grafana
wouldn't have an effect. And the administrators can't change the
automatically imported Grafana dashboards provided by us.

Fixes: https://tracker.ceph.com/issues/51212
Signed-off-by: Patrick Seidensal <pseidensal@suse.com>