Remove recursive call from 32 bit operator-

[mborland]

Closes: #692

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.2%. Comparing base (73669da) to head (14067a4).

Additional details and impacted files

@@            Coverage Diff            @@
##           develop    #693     +/-   ##
=========================================
+ Coverage     99.1%   99.2%   +0.1%     
=========================================
  Files          230     230             
  Lines        16347   16334     -13     
  Branches      1757    1753      -4     
=========================================
- Hits         16199   16188     -11     
+ Misses         148     146      -2     

Files	Coverage Δ
include/boost/decimal/decimal32.hpp	99.8% <100.0%> (-<0.1%)	⬇️
include/boost/decimal/decimal32_fast.hpp	97.1% <100.0%> (-<0.1%)	⬇️
include/boost/decimal/detail/sub_impl.hpp	98.7% <100.0%> (+1.3%)	⬆️

... and 2 files with indirect coverage changes

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