Drop ShellClient::shellSurface method

Summary:
The only place where this method is used is testMaximize. Though that
usage is not justified. We're checking what KWayland already checks.

Test Plan: Compiles, all tests still pass.

Reviewers: KWin, davidedmundson

Reviewed By: KWin, davidedmundson

Subscribers: kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D18635