Remove declarations for things that don't exist.

The <grp.h> ones prevent gdb from building out of the box.

Change-Id: I0efbffad2215cfcd75b4d442dfc972444b51d97c

libstdc++/include/cstdlib

@@ -42,9 +42,6 @@ namespace std {
 using ::exit;
 using ::abort;
 using ::atexit;
-#if 0 /* MISSING FROM BIONIC */
-using ::on_exit;
-#endif
 
 using ::getenv;
 using ::putenv;