[electrum] remove unnecessary hex conversion when converting int to bytes

Summary:
Python int has a builtin to_bytes method.
Also rename int_to_hex to int_to_le_hex to clarify what it is doing.

Test Plan:

python test_runner.py
python -m electrumabc.bitcoin   # run doctest

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14462