[wayland] Discard Force Temporarily rules for shell clients

Summary:
Force Temporarily rules have to be discarded when a client is unmapped.
Otherwise there won't be a difference between Force and Force
Temporarily rules.

Reviewers: #kwin

Subscribers: graesslin, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D18494

shell_client.cpp

@@ -399,6 +399,10 @@ void ShellClient::destroyClient()
         del = Deleted::create(this);
     }
     emit windowClosed(this, del);
+
+    // Remove Force Temporarily rules.
+    RuleBook::self()->discardUsed(this, true);
+
     destroyWindowManagementInterface();
     destroyDecoration();