scripts: Remove unused variables in all Python scripts

Discovered with pylint3.

Use the placeholder name '_' for unproblematic unused variables. It's
what I'm used to, and pylint knows not to flag it.

Python tip:

    for i in range(n):
        some_list.append(0)

can be replaced with

    some_list += n*[0]

Similarly, 3*'\t' gives '\t\t\t'.

(Relevant here because pylint flagged the loop index as unused.)

To do integer division in Python 3, use // instead of /.

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>