Remove header_read_timeout_buffered_io feature flag

What does this MR do and why?

Removes the header_read_timeout_buffered_io feature flag. The flag has been default enabled in %14.9

• Rollout issue: #350233 (closed)
• Original security issue (confidential): #336999 (closed)
• Introducing MR: !78065 (merged)
• Default enabling MR: !81910 (merged)

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