.maintain/monitoring/alerting-rules: Remove HighCPUUsage alert (#6648)

The `HighCPUUsage` alert is based on the `cpu_usage_percentage` metric.
Instead of exposing the overall CPU usage in percent, the metric exposes
the per core usage summed over all cores.

This commit removes the alert for two reasons:

1. Substrate itself does not expose the core count and thus one can not
alert based on the `cpu_usage_percentage` metric.

2. Alerting based on CPU usage is generic and not specific to Substrate
or Blockchains. Thus any CPU usage alert suffice.