Call destroyWindowManagementInterface from ShellClient::destroyClient

Summary:
So far destroyWindowManagementInterface was only called when the
ShellClient got unmapped. But it's possible (although not recommended)
to just destroy the Surface without prior unmapping. In that case the
PlasmaWindow got leaked.

This change addresses this problem by always calling
destroyWindowManagementInterface from ShellClient::destroyClient.

Reviewers: Plasma

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D1762