TXT -> TXT - Trimming lines, commas

filename = "Amplifier_Mod1_angleS21.TXT"
destinationName = "output.txt"
file = open(filename, 'r')
destination = open(destinationName, 'w')

counterLine = 0
for line in file:
    counterLine += 1
    if counterLine > 3:
        newLine = ''
        for letter in line:
            if letter != ',':
                newLine += letter
        destination.write(newLine)