Removed code used when NXT_HAVE_POSIX_SPAWN is false.

posix_spawn(3POSIX) was introduced by POSIX.1d
(IEEE Std 1003.1d-1999), and was later consolidated in
POSIX.1-2001, requiring it in all POSIX-compliant systems.
It's safe to assume it's always available, more than 20 years
after its standardization.

Link: <https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/spawn.h.html>

auto/unix

@@ -178,21 +178,6 @@ if [ $nxt_found = no ]; then
 fi
 
 
-nxt_feature="posix_spawn()"
-nxt_feature_name=NXT_HAVE_POSIX_SPAWN
-nxt_feature_run=
-nxt_feature_incs=
-nxt_feature_libs=
-nxt_feature_test="#include <spawn.h>
-                  #include <unistd.h>
-
-                  int main(int argc, char *argv[]) {
-                      (void) posix_spawn(NULL, \"\", NULL, NULL, argv, NULL);
-                      return 0;
-                  }"
-. auto/feature
-
-
 # NetBSD 1.0, OpenBSD 1.0, FreeBSD 2.2 setproctitle().
 
 nxt_feature="setproctitle()"