Patch from Andrey Legayev

Add #ifndef QT_NO_SYSTEMTRAYICON to KleopatraApplication::~Private()

Summary:
I've found that class member `sysTray` is not created if QT_NO_SYSTEMTRAYICON defined: see it in [[ https://phabricator.kde.org/source/kleopatra/browse/master/src/kleopatraapplication.>
I've added the same pre-processing macros for delete statement to prevent compilation error.

Test Plan: Try to compile with QT_NO_SYSTEMTRAYICON - it should proceed without errors.

Reviewers: mlaurent

Reviewed By: mlaurent

Subscribers: kde-pim

Tags: #kde_pim, #kde_pim_kleopatra

Differential Revision: https://phabricator.kde.org/D28591