[electrum] remove redundant parameters for command-line arguments

Summary:
The dest parameter is inferred from the first long option string, so no need to specify if when they are identical (--offline -> offline). See https://docs.python.org/3/library/argparse.html#dest

The store_true / store_false actions create default values False / True, so no need to specify the default argument when this is what we want.

This will avoid potential bugs wnd extra work when changing the command line arguments.

Test Plan: python -m electrumabc.tests.test_commands

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14148