Handle remove receivergroups

[pepov]

Describe the bug
Resources for receiverGroups are currently not removed when a receiverGroup is removed from the CR. This is becase the receiverGroup is an array and some extra logic would be required to detect and delete resources for groups that are not available anymore in the configuration.

Steps to reproduce the issue:
Create a receiver with two groups, like in the example:
config/samples/monitoring_v1alpha1_receiver.yaml

Expected behavior
The resources that belong to the group are removed.

Additional context
A potential solution would be to use the component reconciler from operator-tools with the receiver as the parent object.