[wayland] Drop initSurface method in XdgShellClient

Summary:
XdgShellClient can represent either a xdg-toplevel or xdg-popup. Therefore,
template initSurface method is no longer needed.

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D24196