Use new xkbcommon API for consumed modifiers

Summary:
This change removes the workaround added with
421824b654a5301fe1c0ed97caded66936c91385.

Xkbcommon 0.7 introduces the needed API to make this work without a
workaround.

Test Plan: Test case still passes

Reviewers: KWin, Plasma on Wayland

Subscribers: plasma-devel, kwin

Tags: Plasma on Wayland, KWin

Differential Revision: https://phabricator.kde.org/D3809