ssh/tailssh: do not send EOT on session disconnection

This was assumed to be the fix for mosh not working, however turns out
all we really needed was the duplicate fd also introduced in the same
commit (af412e8874).

Fixes #5103

Signed-off-by: Maisem Ali <maisem@tailscale.com>

ssh/tailssh/tailssh.go

@@ -1037,9 +1037,6 @@ func (ss *sshSession) run() {
 		if _, err := io.Copy(rec.writer("i", ss.stdin), ss); err != nil {
 			logf("stdin copy: %v", err)
 			ss.ctx.CloseWithError(err)
-		} else if ss.ptyReq != nil {
-			const EOT = 4 // https://en.wikipedia.org/wiki/End-of-Transmission_character
-			ss.stdin.Write([]byte{EOT})
 		}
 	}()
 	go func() {