ModelicaInternal.c: Inconsistency between `_mkdir`/`_chdir` and `_rmdir`

[beutlich]

I also noticed that even if _rmdir is available in MinGW header direct.h, ModelicaInternal.c uses rmdir in line 388 and not _rmdir of line 390. Hence, it is not consistent between _mkdir/_chdir and _rmdir.

https://github.com/modelica/ModelicaStandardLibrary/blob/c2dabe7c979b321760bfc99ad8cc99d2a8777624/Modelica/Resources/C-Sources/ModelicaInternal.c#L385-L393

Originally posted by @beutlich in #4664