[electrum] remove broken PaymentRequestBitPay20

Summary:
I broke the PaymentRequestBitPay20.verify method in D16278 because bitcoin.pubkey_from_signature no longer exists (the function has been moved from bitcoin.py to ecc.py).
This is evidently untested, and I can't find test data to prevent such a regression from occuring again as BitPay currently does not support XEC.

Remove the class since it is unused. It will be easy enough to revert this commit and fix the issue if we ever need to support exotic BitPay bip70 variants (then we will have some data that can be used for test coverage).

Depends on D16352

Test Plan:
check that there are no remaining callsites in the repository for PaymentRequestBitPay20 & get_payment_request_bitpay20
Run the application and test_suite (the regular PaymentRequest & get_payment_request have unit tests) to ensure there is nothing broken due to removing the unused imports.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D16354