PR Description

Remove STBimage preprocessor instruction used to fix MVSC that is no more usefull since DGtal PR 175

Checklist

[ ] Doxygen documentation of the code completed (classes, methods, types, members...)
[ ] Check if it follows the tools structure described in CONTRIBUTING.md
[ ] New entry in the ChangeLog.md added.
[ ] Update the readme with potentially a screenshot of the tools if it applies.
[ ] No warning raised in Debug cmake mode (otherwise, Travis C.I. will fail).
[ ] All continuous integration tests pass (Github Actions & appveyor).