Remove mswin readline legacy code

[alkino]

This copy of readline seems unused today?

[codecov-commenter]

Codecov Report

Merging #1396 (d3754d3) into master (b4da443) will increase coverage by 0.08%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1396      +/-   ##
==========================================
+ Coverage   33.07%   33.15%   +0.08%     
==========================================
  Files         569      569              
  Lines      108959   108959              
==========================================
+ Hits        36037    36125      +88     
+ Misses      72922    72834      -88     

Impacted Files	Coverage Δ
src/parallel/bbs.cpp	64.78% <0.00%> (+1.87%)	⬆️
src/parallel/bbssrv2mpi.cpp	54.59% <0.00%> (+3.44%)	⬆️
src/nrnmpi/bbsmpipack.cpp	86.44% <0.00%> (+12.42%)	⬆️
src/parallel/bbsclimpi.cpp	51.29% <0.00%> (+21.42%)	⬆️
src/parallel/bbssrvmpi.cpp	43.03% <0.00%> (+29.11%)	⬆️

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