Fix renaming of seesions not deleting the old session name

This was caused by the PersistentSettingsWriter writing the old
session on being deleted. Which was after we removed the file.

So delete the PersistentSettingsReader earlier.

Change-Id: I7e73775cc1c2e31b2f2a567b1d08efefdb3ee949
Task-number: QTCREATORBUG-14285
Reviewed-by: Sergey Belyashov <Sergey.Belyashov@gmail.com>
Reviewed-by: Tobias Hunger <tobias.hunger@theqtcompany.com>

src/plugins/projectexplorer/session.cpp

@@ -927,6 +927,8 @@ bool SessionManager::loadSession(const QString &session)
     d->m_values.clear();
 
     d->m_sessionName = session;
+    delete d->m_writer;
+    d->m_writer = 0;
     EditorManager::updateWindowTitles();
 
     if (fileName.exists()) {