GUI: Remove unused menu items for Windows and Linux

Summary:
Remove "Restore" action in the Window menu, and show the "Main Window" action only on Mac.

Backport of Core PR16735

Test Plan:
ninja && ninja check

Run src/qt/bitcoin-qt -regtest and confirm that the menu and remaining actions still work as expected.

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7930