docs: remove the description of port 5558

[orisano]

I remove the description of port 5558.
locust hadn't bind two sockets since from #927

[codecov]

Codecov Report

Merging #1255 into master will increase coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1255      +/-   ##
==========================================
+ Coverage   77.51%   77.56%   +0.04%     
==========================================
  Files          20       20              
  Lines        2006     2006              
  Branches      316      316              
==========================================
+ Hits         1555     1556       +1     
+ Misses        370      369       -1     
  Partials       81       81              

Impacted Files	Coverage Δ
locust/core.py	91.76% <0.00%> (+0.41%)	⬆️

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[heyman]

Thanks!