Codechange: Introduce FioRemove() to remove files. (#12491)

New function FioRemove() handles OTTD2FS conversion, and uses std::filesystem::remove instead of unlink, all in one location.

src/stdafx.h

@@ -169,7 +169,6 @@
 #		include <tchar.h>
 
 #		define fopen(file, mode) _wfopen(OTTD2FS(file).c_str(), _T(mode))
-#		define unlink(file) _wunlink(OTTD2FS(file).c_str())
 
 		std::string FS2OTTD(const std::wstring &name);
 		std::wstring OTTD2FS(const std::string &name);