[electrum] drop support for "bitcoincash:..." addresses

Summary:
After this diff, bch cash addresses are no longer supported in the "Pay to" field, in BIP21 URIs or any method of entering an address that uses Address.from_string without setting support_arbitrary_prefix=True.

The address converter in the GUI still support any prefix. This diff also makes the command line / RPC addressconvert command support arbitrary prefixes.
But now we no longer support bch addresses without an explicit prefix.

This diff also incidentally fixes support for "ectest:..." URIs when running in testnet mode. Previously web.parseable_schemes did not correctly account for the net parameter, and always returned ("ecash", "bitcoincash")

This is a follow-up to https://github.com/Bitcoin-ABC/ElectrumABC/pull/250 (dropping of display of BCH addresses in the UI).

Test Plan:

python test_runner.py
pytest electrumabc.tests.regtest

Try the address converter tool in the prefix with various address formats. It should still support everything it did before, except for prefixless "bitcoincash:..." cash addresses.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D15245