LibC: Remove undefined `__generate_unique_filename` declaration

Userland/Libraries/LibC/stdlib.h

@@ -10,8 +10,6 @@
 #include <sys/cdefs.h>
 #include <sys/types.h>
 
-__attribute__((warn_unused_result)) int __generate_unique_filename(char* pattern);
-
 __BEGIN_DECLS
 
 #define EXIT_SUCCESS 0