[electrum] move DUST_THRESHOLD constant to transaction.py

Summary:
Most modules that use this constant already import stuff from transaction.py, which tells us that it is a good place to define this value.

Depends on D14421

Test Plan:
python test_runner.py

Run the application to detect potential import errors.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14422