Use prometheus' default securityContext
Now that #236 (closed) was solved, we can remove our hotfix from Prometheus' configuration:
# https://github.com/rancher/rancher/issues/14836 prevents 'subPath' to work
# with non-privileged users. This is an insecure workaround for the time
# being...
securityContext:
runAsUser: 0
fsGroup: 0
runAsNonRoot: false