Remove unused abuse_report_labels feature flag and related code

What does this MR do and why?

This merge request removes all code related to the abuse_report_labels feature flag. The feature flag was introduced in %16.4 but never released or used in production.

We confirmed that there are zero rows in the abuse_report_labels table across production environments, indicating that the feature was never adopted:

select * from abuse_report_labels;

 Seq Scan on public.abuse_report_labels  (cost=0.00..0.00 rows=1 width=164) (actual time=0.003..0.004 rows=0 loops=1)
   I/O Timings: read=0.000 write=0.000
Settings: effective_cache_size = '472585MB', jit = 'off', work_mem = '100MB', random_page_cost = '1.5', seq_page_cost = '4'

https://console.postgres.ai/gitlab/gitlab-production-main/sessions/43972/commands/134580

select * from abuse_report_label_links;

 Seq Scan on public.abuse_report_label_links  (cost=0.00..0.00 rows=1 width=40) (actual time=0.003..0.004 rows=0 loops=1)
   I/O Timings: read=0.000 write=0.000
Settings: effective_cache_size = '472585MB', jit = 'off', random_page_cost = '1.5', seq_page_cost = '4', work_mem = '100MB'

https://console.postgres.ai/gitlab/gitlab-production-main/sessions/44146/commands/135080

Breaking change approval for this removal is documented in gitlab-com/Product#14362.

References

• Breaking change exception: Remove abuse_report_... (gitlab-com/Product#14362) • mo khan, Mark Mishaev • On track
• Remove abuse_report_labels Feature Flag (#517640 - closed) • mo khan • 18.6
• Sharding Key Investigation for `abuse_report_la... (#553430 - closed) • mo khan • 18.6 • On track
• Sharding Key Investigation for `abuse_report_la... (#553431 - closed) • mo khan • 18.6 • On track
• Sharding Key Investigation for `abuse_report_la... (#553431 - closed) • mo khan • 18.6 • On track
• Backfill organization_id column in abuse_report... (#562573 - closed) • mo khan • 18.6
• Add sharding_key to abuse_report_labels table d... (#562574 - closed) • mo khan • 18.6
• Set abuse_report_labels.organization_id as not ... (#562575 - closed) • mo khan • 18.6
• Documentation

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