gui: don't disable the sync overlay when wallet is disabled

Summary:
When running with -disablewallet the sync modal is now available by clicking on the progress bar or syncing icon.

This is a backport of Core PR15084

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

Click on the progress bar or syncing icon to show the sync overlay dialog.

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, majcosta

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, majcosta

Subscribers: majcosta

Differential Revision: https://reviews.bitcoinabc.org/D8161