[electrum] update CONTRIBUTING.md

Summary:
The Electrum ABC specific document is no longer relevant, remove it.
Update the main CONTRIBUTING.md doc for Electrum ABC dependencies.

Depends on D14126

Test Plan: try the pip install commands.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14139