Remove u128 x64 intrinsic pessimzations

[mborland]

I never bench-marked these properly and they do in fact make things slightly worse so best if they are removed.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.2%. Comparing base (89e1dd0) to head (c995e23).
Report is 5 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff            @@
##           develop    #725     +/-   ##
=========================================
+ Coverage     99.1%   99.2%   +0.1%     
=========================================
  Files          229     224      -5     
  Lines        16430   16375     -55     
  Branches      1781    1778      -3     
=========================================
- Hits         16276   16233     -43     
+ Misses         154     142     -12     

Files with missing lines	Coverage Δ
include/boost/decimal/detail/emulated128.hpp	97.6% <100.0%> (-<0.1%)	⬇️

... and 10 files with indirect coverage changes

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