kafka: remove `messaging.kafka.connect_errors.count` metric

[endorama]

In #479 we introduced separate dimensions on messaging.kafka.connects.count to track successes and failures for connections to Kafka.

To prevent a breaking change we did not remove messaging.kafka.connect_errors.count, the metric we inherited from franz-go kotel plugin, with the plan to remove it at a later moment.

This issue is here to keep track of its removal.

[simitt]

@endorama how long do we need to keep the messaging.kafka.connect_errors.count metric ?

[endorama]

I identified an issue with using KQL filters in alerts that could force us to use 2 metrics until it's fixed. At the moment we can say that this metric cannot be removed safely.