chore(3p/nix): Remove some OS X specific defines

This project will be dropping OS X support until the core is simplified.

third_party/nix/src/libmain/shared.cc

@@ -142,17 +142,6 @@ void initNix() {
     throw SysError("handling SIGUSR1");
   }
 
-#if __APPLE__
-  /* HACK: on darwin, we need can’t use sigprocmask with SIGWINCH.
-   * Instead, add a dummy sigaction handler, and signalHandlerThread
-   * can handle the rest. */
-  struct sigaction sa;
-  sa.sa_handler = sigHandler;
-  if (sigaction(SIGWINCH, &sa, 0)) {
-    throw SysError("handling SIGWINCH");
-  }
-#endif
-
   /* Register a SIGSEGV handler to detect stack overflows. */
   detectStackOverflow();
 
@@ -165,14 +154,6 @@ void initNix() {
   struct timeval tv;
   gettimeofday(&tv, nullptr);
   srandom(tv.tv_usec);
-
-  /* On macOS, don't use the per-session TMPDIR (as set e.g. by
-     sshd). This breaks build users because they don't have access
-     to the TMPDIR, in particular in ‘nix-store --serve’. */
-#if __APPLE__
-  if (getuid() == 0 && hasPrefix(getEnv("TMPDIR"), "/var/folders/"))
-    unsetenv("TMPDIR");
-#endif
 }
 
 LegacyArgs::LegacyArgs(