Kernel: Remove "restorer" field from SignalActionData.

I was originally implementing signals by looking at some man page about
sigaction() to see how it works. It seems like the restorer thingy is
system-specific and not required by POSIX, so let's get rid of it.

LibC/signal.cpp

@@ -33,7 +33,6 @@ sighandler_t signal(int signum, sighandler_t handler)
     new_act.sa_handler = handler;
     new_act.sa_flags = 0;
     new_act.sa_mask = 0;
-    new_act.sa_restorer = nullptr;
     int rc = sigaction(signum, &new_act, &old_act);
     if (rc < 0)
         return SIG_ERR;

LibC/signal.h

@@ -19,7 +19,6 @@ struct sigaction {
     };
     sigset_t sa_mask;
     int sa_flags;
-    void (*sa_restorer)(void);
 };
 
 int kill(pid_t, int sig);