Delete external tools on shutdown

Fixes memory leak:
==7961== 5,204 (864 direct, 4,340 indirect) bytes in 9 blocks are definitely lost in loss record 1,148 of 1,189
==7961==    at 0x4836DEF: operator new(unsigned long) (in /usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)
==7961==    by 0x109836E7: KateExternalToolsPlugin::reload() (externaltoolsplugin.cpp:102)
==7961==    by 0x1098328B: KateExternalToolsPlugin::KateExternalToolsPlugin(QObject*, QList<QVariant> const&) (externaltoolsplugin.cpp:68)
==7961==    by 0x109872C9: QObject* KPluginFactory::createInstance<KateExternalToolsPlugin, QObject>(QWidget*, QObject*, QList<QVariant> const&) (kpluginfactory.h:533)
==7961==    by 0x6880B07: KPluginFactory::create(char const*, QWidget*, QObject*, QList<QVariant> const&, QString const&) (kpluginfactory.cpp:145)
==7961==    by 0x46D00A: KTextEditor::Plugin* KPluginFactory::create<KTextEditor::Plugin>(QObject*, QList<QVariant> const&) (kpluginfactory.h:551)
==7961==    by 0x46BD81: KatePluginManager::loadPlugin(KatePluginInfo*) (katepluginmanager.cpp:198)
==7961==    by 0x46B6E1: KatePluginManager::loadConfig(KConfig*) (katepluginmanager.cpp:134)
==7961==    by 0x4A3065: KateSessionManager::loadSession(QExplicitlySharedDataPointer<KateSession> const&) const (katesessionmanager.cpp:183)
==7961==    by 0x4A2ED0: KateSessionManager::activateSession(QExplicitlySharedDataPointer<KateSession>, bool, bool) (katesessionmanager.cpp:166)
==7961==    by 0x4A360F: KateSessionManager::activateSession(QString const&, bool, bool) (katesessionmanager.cpp:242)
==7961==    by 0x4A44F3: KateSessionManager::chooseSession() (katesessionmanager.cpp:409)