Remove unused CounterAttribute methods

What does this MR do and why?

Removes unused methods from the CounterAttribute concern:

• update_counters_with_lease - Primary target (public method, never called)
• detect_race_on_record - Cascade removal (private, only called by above)
• database_lock_key - Cascade removal (private, only called by above)
• include Gitlab::ExclusiveLeaseHelpers - Unused after method removals

These methods were identified as unused by scripts/lint/unused_code.rb and verified through comprehensive codebase search including metaprogramming patterns.

Related to #542966 (closed)

Metrics

Metric	Value
Methods removed	3
Lines removed (source)	48
Lines removed (spec)	50
Lines removed (config)	2
Total lines removed	100
Spec runtime saved	8.91 seconds per pipeline

Estimated CI Savings

Estimate	Pipelines/Month	Time Saved/Month	Cost Saved/Month	Cost Saved/Year
Pessimistic	15,000	37.1 hrs	$66.83	$801.90
Moderate	30,000	74.3 hrs	$133.65	$1,603.80
Optimistic	50,000	123.8 hrs	$222.75	$2,673.00

Based on 8.91 seconds saved per pipeline × $0.03/minute (large runners: 3x cost factor on $10/1000 base minutes)

Verification

• No direct callers found (excluding specs)
• No metaprogramming references (send, public_send, method)
• No symbol references (:update_counters_with_lease)
• No Rails callbacks using these methods
• No delegation patterns
• All remaining specs pass (67 examples, 0 failures)
• Rubocop clean

MR acceptance checklist

• Tests removed for deleted functionality
• Changelog entry added

Related

• Parent issue: #542966 (closed) (Dead code removal initiative)