capi: fix Windows build after PR 2201

[canepat]

This PR fix Windows build after #2201.

The error was misleading because MSVC 19.40 complains about Compiler Error C2440, which seems strange at first, but in the end it's due to the missing conversion operator from std::filesystem::path to std::string.

Extras

• remove unused include
• simplify some expressions as suggested by CLion

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 53.51%. Comparing base (8ef63c7) to head (7753506).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2216   +/-   ##
=======================================
  Coverage   53.50%   53.51%           
=======================================
  Files         689      689           
  Lines       48046    48046           
  Branches     7292     7292           
=======================================
+ Hits        25708    25710    +2     
+ Misses      20087    20084    -3     
- Partials     2251     2252    +1     

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