terminal-util: drop support for pre-TIOCGPTPEER kernels

Our minimum baseline is now far beyond 4.13, hence let's drop these
fallback paths.

src/basic/terminal-util.c

@@ -2324,7 +2324,7 @@ int terminal_is_pty_fd(int fd) {
         return true;
 }
 
-int pty_open_peer_racefree(int fd, int mode) {
+int pty_open_peer(int fd, int mode) {
         assert(fd >= 0);
 
         /* Opens the peer PTY using the new race-free TIOCGPTPEER ioctl() (kernel 4.13).
@@ -2339,9 +2339,6 @@ int pty_open_peer_racefree(int fd, int mode) {
                 if (peer_fd >= 0)
                         return peer_fd;
 
-                if (ERRNO_IS_IOCTL_NOT_SUPPORTED(errno)) /* new ioctl() is not supported, return a clear error */
-                        return -EOPNOTSUPP;
-
                 if (errno != EIO)
                         return -errno;
 
@@ -2352,31 +2349,3 @@ int pty_open_peer_racefree(int fd, int mode) {
                 (void) usleep_safe(50 * USEC_PER_MSEC);
         }
 }
-
-int pty_open_peer(int fd, int mode) {
-        int r;
-
-        assert(fd >= 0);
-
-        /* Opens the peer PTY using the new race-free TIOCGPTPEER ioctl() (kernel 4.13) if it is
-         * available. Otherwise falls back to the POSIX ptsname() + open() logic.
-         *
-         * Because of the fallback path this is not safe to be called on PTYs from other namespaces. (Because
-         * we open the peer PTY name there via a path in the file system.) */
-
-        // TODO: Remove fallback path once baseline is updated to >= 4.13, i.e. systemd v258
-
-        int peer_fd = pty_open_peer_racefree(fd, mode);
-        if (peer_fd >= 0)
-                return peer_fd;
-        if (!ERRNO_IS_NEG_NOT_SUPPORTED(peer_fd))
-                return peer_fd;
-
-        /* The racy fallback path */
-        _cleanup_free_ char *peer_path = NULL;
-        r = ptsname_malloc(fd, &peer_path);
-        if (r < 0)
-                return r;
-
-        return open_terminal(peer_path, mode);
-}

src/basic/terminal-util.h

@@ -165,7 +165,6 @@ int terminal_fix_size(int input_fd, int output_fd);
 
 int terminal_is_pty_fd(int fd);
 
-int pty_open_peer_racefree(int fd, int mode);
 int pty_open_peer(int fd, int mode);
 
 static inline bool osc_char_is_valid(char c) {