gnuplot - removing line title

If you had more untitled lines than titled lines, it's more convenient to disable titles by default using set key noautotitle:

set key noautotitle

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"

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 ""