Fix deletequick() and deletetele() not deleting their files

The problem here is that we're directly using the C stdio library,
instead of using PHYSFS's stuff. So I've added a function
FILESYSTEM_delete() that does exactly that.

desktop_version/src/FileSystemUtils.h

@@ -23,4 +23,6 @@ std::vector<std::string> FILESYSTEM_getLevelDirFileNames();
 bool FILESYSTEM_openDirectoryEnabled();
 bool FILESYSTEM_openDirectory(const char *dname);
 
+bool FILESYSTEM_delete(const char *name);
+
 #endif /* FILESYSTEMUTILS_H */