Remove unnecessary writeHTTPParts allocation

[adam-fowler]

Don't use result from writeAndFlush so don't create promise for it. Return static successful void future

[codecov]

Codecov Report

Merging #30 (ab12c64) into main (6795503) will increase coverage by 0.18%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #30      +/-   ##
==========================================
+ Coverage   82.53%   82.72%   +0.18%     
==========================================
  Files          19       19              
  Lines         733      735       +2     
==========================================
+ Hits          605      608       +3     
+ Misses        128      127       -1     

Impacted Files	Coverage Δ
...ces/HummingbirdCore/Server/HTTPServerHandler.swift	84.61% <100.00%> (+0.72%)	⬆️

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