Merge #12721: Qt: remove "new" button during receive-mode in addressbook

Summary:
d843db7 Qt: remove "new" button during receive-mode in addressbook (Jonas Schnelli)

Pull request description:

There are currently two ways how to generate new receiving addresses in the GUI (which leads to code duplication or required refactoring, see #12520).

Since the address-book is probably something that should be removed in the long run, suppressing the new-button in receive-mode could be a first step in deprecating the address book.

With this PR, users can still edit existing receiving address book entries and they can still create new sending address book entries.

Tree-SHA512: abe8d1b44bc3e1b53826ccf9d2b3f764264337758d95ca1fe1ef1bac72d47608cf454055fce3720e06634f0a5841a752ce643b4505b47d6e322b6fc71296e961

Backport of Core PR12721
https://github.com/bitcoin/bitcoin/pull/12721/

Test Plan:

make check
./bitcoin-qt -> file -> receiving addresses

Verify there is no new button on the bottom left side

Reviewers: deadalnix, Fabien, jasonbcox, O1 Bitcoin ABC, #bitcoin_abc

Reviewed By: Fabien, O1 Bitcoin ABC, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D3941