Remove now unused d128 addition impl

[mborland]

No description provided.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.0%. Comparing base (1def358) to head (21f218e).
⚠️ Report is 10 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff            @@
##           develop   #1297     +/-   ##
=========================================
+ Coverage     98.9%   99.0%   +0.1%     
=========================================
  Files          270     270             
  Lines        17956   17910     -46     
  Branches      1998    1973     -25     
=========================================
- Hits         17742   17714     -28     
+ Misses         214     196     -18     

Files with missing lines	Coverage Δ
include/boost/decimal/detail/add_impl.hpp	98.0% <ø> (+12.5%)	⬆️
include/boost/decimal/detail/cmath/fma.hpp	100.0% <100.0%> (ø)
include/boost/decimal/detail/u256.hpp	100.0% <100.0%> (ø)

... and 2 files with indirect coverage changes

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