Draft: Error Tracking: Consolidate feature flag "integrated_error_tracking"

What does this MR do and why?

The feature flag integrated_error_tracking is only being used in with combination gitlab_error_tracking. At this point, we decided to use Clickhouse so there's no need to have a separate feature flag.

Note that this feature is enabled on GitLab.com only and the feature flag(s) are both enabled.

Self-managed users are unaffected.

Contributes to #353956.

Follow-up of !122869 (merged).

feature flag cleanup is tracked in #353956 (comment 1421615435).

How to set up and validate locally

In Rails console:

Feature.enable(:gitlab_error_tracking)
Feature.remove(:integrated_error_tracking)

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.