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>

src/nxt_unix.h

@@ -156,9 +156,7 @@
 #include <setjmp.h>
 #include <sched.h>
 #include <signal.h>
-#if (NXT_HAVE_POSIX_SPAWN)
 #include <spawn.h>
-#endif
 #include <stdarg.h>
 #include <stddef.h>                 /* offsetof() */
 #include <stdio.h>