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.

Userland/sh.cpp

@@ -232,7 +232,6 @@ int main(int argc, char** argv)
         sa.sa_handler = handle_sigint;
         sa.sa_flags = 0;
         sa.sa_mask = 0;
-        sa.sa_restorer = nullptr;
         int rc = sigaction(SIGINT, &sa, nullptr);
         assert(rc == 0);
     }