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.

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