Remove EnsureStageService

What does this MR do and why?

Info below from !153259 (closed), which was closed in lieu of this MR.

This MR removes the EnsureStageService and the feature flag. Notes:

• Feature flag has been enabled globally more than a year with no issues.
• It was enabled for Self-Managed during 17.1
• We have model level validation for ci_stage in CommitStatus, which is the parent class for GenericCommitStatus and Ci::Processable, which in turn is the parent class for Ci::Build and Ci::Bridge.
• GenericCommitStatus, which is used in UpdatePagesService is already enforcing a stage, and it will always place the generic commit status at the end via the EXTERNAL_STAGE_IDX, which is set to 1_000_000.
• The default stage test is set in Ci::Config::Entry::Stage

Initially, the intention before the removal of the service was that we should add a not null constraint to the stage_id column in the ci_builds table, but this has proven to be too costly and I don't think it's worth it. The issue is that we have a few records in production without a stage_id, and we needed to take care of them first before enforcing this validation at database level. The only way to take care of these records is via a batched background migration, but given that the table is partitioned AND it's got millions of records, this operation is waaaay too costly. As a rough plan, it would require adding an async index, doing a batched migration which will take forever, and then all the subsequent work of cleanup. We already have a model-level validation which will prevent new null values from being inserted. Convo I had about this here: https://gitlab.slack.com/archives/C3NBYFJ6N/p1716903893212369 (internal)

Summary, TLDR: Remove EnsureStageService

Changelog: removed

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#364377