[electrum] cleanup ECC_YfromX

Summary:
Rename the function to get_y_coord_from_x, return only y (the offset is ignored in the only callsite), don't pass the curve as a parameter (we always use the same one).

This is split out for D16301

Test Plan: python test_runner.py

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D16347