http: remove redundant keepAliveTimeoutBuffer assignment

[efekrskl]

storeHTTPOptions() (see lib/_http_server.js:496) already validates and initializes keepAliveTimeoutBuffer. This PR removes the redundant assignment so option validation and defaulting have a single source of truth.

[nodejs-github-bot]

Review requested:

• @nodejs/http
• @nodejs/net

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.73%. Comparing base (1e8a639) to head (4e7c085).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #61743   +/-   ##
=======================================
  Coverage   89.73%   89.73%           
=======================================
  Files         675      675           
  Lines      204525   204519    -6     
  Branches    39314    39304   -10     
=======================================
+ Hits       183523   183529    +6     
+ Misses      13300    13293    -7     
+ Partials     7702     7697    -5     

Files with missing lines	Coverage Δ
lib/_http_server.js	97.17% <ø> (+0.27%)	⬆️

... and 23 files with indirect coverage changes

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[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/71263/

[mcollina]

lgtm