west: boards: Remove accidental comma to fix pylint warning

Remove a trailing comma that generated a single-element tuple and made
pylint warn:

    scripts/west_commands/boards.py:50:8: W0106: Expression
    "(parser.add_argument(...), )" is assigned to nothing
    (expression-not-assigned)

No functional change.

Fixing pylint warnings for a CI check.

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