Fix a crash on codestylesettings

Task-number: QTCREATORBUG-10235

Instead of rely on ICodeStylePreferences::destroyed() signal,
when all other objects might be in destruction phase,
clear project code style settings explicitly, when project
closes.

Change-Id: I0dd6675d54c5495d4006acbc9ad12c95f1d0a00c
Reviewed-by: Eike Ziller <eike.ziller@digia.com>
Reviewed-by: Jarek Kobus <jaroslaw.kobus@digia.com>