Remove pipeline_name_search feature flag

What does this MR do and why?

Feature flag pipeline_name_search has been enabled on gitlab.com for many months now (enabled in June). This MR removes the feature flag.

Feature flag rollout issue: #385864 (closed)

Performance of the query behind the FF: https://console.postgres.ai/gitlab/gitlab-production-ci/sessions/24980/commands/79384

Query

SELECT
    "ci_pipelines"."id",
    "ci_pipelines"."ref",
    "ci_pipelines"."sha",
    "ci_pipelines"."before_sha",
    "ci_pipelines"."created_at",
    "ci_pipelines"."updated_at",
    "ci_pipelines"."tag",
    "ci_pipelines"."yaml_errors",
    "ci_pipelines"."committed_at",
    "ci_pipelines"."project_id",
    "ci_pipelines"."status",
    "ci_pipelines"."started_at",
    "ci_pipelines"."finished_at",
    "ci_pipelines"."duration",
    "ci_pipelines"."user_id",
    "ci_pipelines"."lock_version",
    "ci_pipelines"."pipeline_schedule_id",
    "ci_pipelines"."source",
    "ci_pipelines"."config_source",
    "ci_pipelines"."protected",
    "ci_pipelines"."failure_reason",
    "ci_pipelines"."iid",
    "ci_pipelines"."merge_request_id",
    "ci_pipelines"."source_sha",
    "ci_pipelines"."target_sha",
    "ci_pipelines"."external_pull_request_id",
    "ci_pipelines"."ci_ref_id",
    "ci_pipelines"."locked",
    "ci_pipelines"."partition_id",
    "ci_pipelines"."auto_canceled_by_id"
FROM
    "ci_pipelines"
    INNER JOIN "ci_pipeline_metadata" ON "ci_pipeline_metadata"."pipeline_id" = "ci_pipelines"."id"
WHERE
    "ci_pipelines"."project_id" = 278964
    AND "ci_pipelines"."source" != 12
    AND "ci_pipeline_metadata"."name" = 'Ruby 3.0 merged_result MR pipeline (community contribution)'
ORDER BY
    "ci_pipelines"."id" DESC
LIMIT 20 OFFSET 0

Query performance

 Limit  (cost=1.13..15724.35 rows=7 width=361) (actual time=0.287..0.588 rows=20 loops=1)
 Buffers: shared hit=104 read=4
 I/O Timings: read=0.110 write=0.000
 ->  Nested Loop  (cost=1.13..15724.35 rows=7 width=361) (actual time=0.286..0.584 rows=20 loops=1)
       Buffers: shared hit=104 read=4
       I/O Timings: read=0.110 write=0.000
       ->  Index Only Scan Backward using index_pipeline_metadata_on_name_pipeline_id_text_pattern on public.ci_pipeline_metadata  (cost=0.56..140.88 rows=4333 width=8) (actual time=0.171..0.174 rows=20 loops=1)
             Index Cond: (ci_pipeline_metadata.name = 'Ruby 3.0 merged_result MR pipeline (community contribution)'::text)
             Heap Fetches: 0
             Buffers: shared hit=1 read=4
             I/O Timings: read=0.110 write=0.000
       ->  Index Scan using index_ci_pipelines_on_project_id_and_id_desc on public.ci_pipelines  (cost=0.57..3.60 rows=1 width=361) (actual time=0.020..0.020 rows=1 loops=20)
             Index Cond: ((ci_pipelines.project_id = 278964) AND (ci_pipelines.id = ci_pipeline_metadata.pipeline_id))
             Filter: (ci_pipelines.source <> 12)
             Rows Removed by Filter: 0
             Buffers: shared hit=103
             I/O Timings: read=0.000 write=0.000

Time: 11.237 ms
  - planning: 10.518 ms
  - execution: 0.719 ms
  - I/O read: 0.110 ms
  - I/O write: 0.000 ms

Shared buffers:
  - hits: 104 (~832.00 KiB) from the buffer pool
  - reads: 4 (~32.00 KiB) from the OS file cache, including disk I/O
  - dirtied: 0
  - writes: 0

Screenshots or screen recordings

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Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

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