tree-wide: drop support for kernels without pidfd_open() and pidfd_send_signal()

Our baseline is v5.4 now.

src/basic/missing_syscall.h

@@ -299,12 +299,7 @@ static inline long missing_get_mempolicy(int *mode, unsigned long *nodemask,
 
 #if !HAVE_PIDFD_SEND_SIGNAL
 static inline int missing_pidfd_send_signal(int fd, int sig, siginfo_t *info, unsigned flags) {
-#  ifdef __NR_pidfd_send_signal
         return syscall(__NR_pidfd_send_signal, fd, sig, info, flags);
-#  else
-        errno = ENOSYS;
-        return -1;
-#  endif
 }
 
 #  define pidfd_send_signal missing_pidfd_send_signal
@@ -312,12 +307,7 @@ static inline int missing_pidfd_send_signal(int fd, int sig, siginfo_t *info, un
 
 #if !HAVE_PIDFD_OPEN
 static inline int missing_pidfd_open(pid_t pid, unsigned flags) {
-#  ifdef __NR_pidfd_open
         return syscall(__NR_pidfd_open, pid, flags);
-#  else
-        errno = ENOSYS;
-        return -1;
-#  endif
 }
 
 #  define pidfd_open missing_pidfd_open