[Qt] Remove unused WindowFlags parameters

Summary:
The WindoFlags parameter is never set in the codebase, and trigger
deprecation warnings with Qt 5.15.

Test Plan:
With Qt 5.5.1 and Qt 5.15:

ninja all check

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6572