Removed instance level serverless domains feature

What does this MR do and why?

The instance level serverless domain feature was developed under the feature flag but wasn't enabled for the users yet (#35591 (closed)). This MR removes the instance level serverless domain page (available at admin -> settings -> operations) together with the corresponding specs and :serverless_domain feature flag.

We won't need the feature anymore as Serverless is deprecated and will be removed in %15.0, see gitlab-org/configure&6 (closed).

The MR is related to #254266 (closed)

Screenshots or screen recordings

The screen that is removed:

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