Remove unused SENTINELAUTH placeholder and related dead code

[coderabbitai]

Background

The SENTINELAUTH feature and related placeholder code in Redis init scripts were never fully implemented or advertised, and should be removed as dead code.

Details

• The __REPLACE_DEFAULT_SENTINEL_AUTH__ placeholder is referenced in build/redis/init.sh.tpl but has no corresponding template configuration
• The original implementation (commit 7d0356b) never completed the feature
• No public documentation or usage of SENTINELAUTH support exists
• The code has been carried forward without being functional

Affected Files

• build/redis/init.sh.tpl (lines around 268 with SENTINELAUTH substitution logic)
• Any other references to SENTINELAUTH or REPLACE_DEFAULT_SENTINEL_AUTH

Acceptance Criteria

• Remove SENTINELAUTH-related code from init scripts
• Remove unused placeholder references
• Clean up any related dead code

References

• PR: feat(redis): Inject creds via volume #2059
• Comment: feat(redis): Inject creds via volume #2059 (comment)
• Original commit: 7d0356b
• Reported by: @olivergondza