[electrum] remove bip70 script

Summary:
This script no longer works since the paymentrequest API changed, and it's not worth the effort fixing it as it no longer needed and its output is no longer supported.
Loading paymentrequests from file is no longer supported since https://github.com/Bitcoin-ABC/ElectrumABC/commit/42240f9e1494dd00023920137e605bf25c6e7df9, as accepting URIs with a "file:/" scheme from remote servers was a security issue on Windows.

Test Plan: NA.

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D15362