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.

Kernel/Process.cpp

@@ -1601,10 +1601,8 @@ int Process::sys$sigaction(int signum, const sigaction* act, sigaction* old_act)
         if (!validate_write_typed(old_act))
             return -EFAULT;
         old_act->sa_flags = action.flags;
-        old_act->sa_restorer = (decltype(old_act->sa_restorer))action.restorer.get();
         old_act->sa_sigaction = (decltype(old_act->sa_sigaction))action.handler_or_sigaction.get();
     }
-    action.restorer = LinearAddress((dword)act->sa_restorer);
     action.flags = act->sa_flags;
     action.handler_or_sigaction = LinearAddress((dword)act->sa_sigaction);
     return 0;

Kernel/Thread.h

@@ -21,7 +21,6 @@ struct SignalActionData {
     LinearAddress handler_or_sigaction;
     dword mask { 0 };
     int flags { 0 };
-    LinearAddress restorer;
 };
 
 class Thread : public InlineLinkedListNode<Thread> {

Kernel/UnixTypes.h

@@ -246,7 +246,6 @@ struct sigaction {
     };
     sigset_t sa_mask;
     int sa_flags;
-    void (*sa_restorer)(void);
 };
 
 #define SA_NOCLDSTOP 1