To accomplish this you should use the notitle tag.
plot "delay_try1.dat" using 1:2 title 'With CloneScale' with lines, "normal_2.dat" using 1:2 title "Without CloneScale" with lines lc rgb "black", "normal2.dat" using 1:2 with lines lc rgb "black" notitle
or a more general example;
plot 'File.dat' using 1:2 notitle
an alternative that is equivalent to notitle is to set the title to a zero character string;
plot 'File.dat' using 1:2 title ""