Remove superflous assignment to exponent argument in parser

[Flamefire]

The exponent will be parsed later, the value assigned at this position is meaningless or even wrong.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.76%. Comparing base (8fbdb8a) to head (9e4975c).
Report is 5 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #240      +/-   ##
===========================================
- Coverage    94.76%   94.76%   -0.01%     
===========================================
  Files           69       69              
  Lines         9031     9030       -1     
===========================================
- Hits          8558     8557       -1     
  Misses         473      473              

Files with missing lines	Coverage Δ
include/boost/charconv/detail/parser.hpp	79.68% <ø> (-0.11%)	⬇️

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