[electrum] remove unused Transaction.is_memory_compact

Summary:
This is unused since https://github.com/Bitcoin-ABC/ElectrumABC/commit/93f07bc7b162b95ce39ad549b8ca0892ab679ac4
and it is wrong since we updated the default transaction version to 2.

Test Plan:
python test_runner.py

grep -r is_memory_compact .

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D14460