Clean systray icon menu for -disablewallet mode

Summary:

This is a backport of Core PR14383 and PR15023

PR 14383 description:

There is a Debug window leftover in the system tray icon menu after PR3392 merging.
This PR makes both the app menu and the systray icon menu consistent.

PR15023 description:

PR14383 moved the debug window's menu position, to make it conditional on wallet mode. The rationale given was to match the behaviour of the 'Help' menu.

PR14573 replaced the 'Help' menu's conditional debug window with an unconditional list of items in the new 'Window' menu.

This PR reverts the no-longer-applicable part of #14383, putting the debug window back on the tray menu unconditionally, and in the position it previously had.

Note: PR14573 was backported in D7892

Test Plan: ninja && src/qt/bitcoin-qt -disablewallet

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Subscribers: deadalnix, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D8146