Remove gspawn_win32_helper executable
Nowdays, gspawn_win32_helper might not be needed anymore.
Since Vista, there are the APIs that work around the limitations of the previous OSes, and we may not need a helper executable anymore for the g_spawn family of APIs.
This would help for static builds on Windows (and deployment on Windows, in general), since Meson does not let us set environment variables, and for static builds we should copy the helper executable down all subfolders of the Meson build folder.