Remove example sensu rule

[Kami]

TODO:

• Update docs (docs/source/resources/monitoring.rst)

As an alternative, we can modify packaging installation steps to not copy this rule over to /opt/stackstorm/packs

[dzimine]

Thanks!
Let's just update the dosc by writing down the rule into the .rst instead of linking to the file;
and point out to the rule samples in the sensu pack (just mention "rule samples available in the pack /opt/stackstorm/packs/sensu/rules (link github/st2contrib/sensu/packs/sensu/rules)

[Kami]

K, sounds good.