Remove the deprecated address_type field from the getnewaddress RPC

Summary:
This field was causing confusion because the "legacy" output type doesn't lead to the generation of a legacy (aka btc) address. This has been deprecated since 0.30.4 so it's time to remove it.

Depends on D17908.

Test Plan:

ninja all check-extended

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D17909