Fix memory leak

NewCountNotifier was never deleted because it never emitted any signals:

==26643== 480 (40 direct, 440 indirect) bytes in 1 blocks are definitely lost in loss record 2,082 of 2,146
==26643==    at 0x4838424: operator new(unsigned int) (vg_replace_malloc.c:292)
==26643==    by 0x4DCD80F: QMetaObjectPrivate::connect(QObject const*, int, QMetaObject const*, QObject const*, int, QMetaObject const*, int, int*) (qobject.cpp:3141)
==26643==    by 0x4DD0C47: QObject::connect(QObject const*, char const*, QObject const*, char const*, Qt::ConnectionType) (qobject.cpp:2610)
==26643==    by 0x18CA3: MessageServer::reportNewCounts() (messageserver.cpp:380)
==26643==    by 0x50A17: MessageServer::qt_static_metacall(QObject*, QMetaObject::Call, int, void**) (moc_messageserver.cpp:126)
==26643==    by 0x4DCC43F: QMetaObject::activate(QObject*, int, int, void**) (qobject.cpp:3580)
==26643==    by 0x5255B: ServiceHandler::qt_static_metacall(QObject*, QMetaObject::Call, int, void**) (moc_servicehandler.cpp:509)
==26643==    by 0x4DCC43F: QMetaObject::activate(QObject*, int, int, void**) (qobject.cpp:3580)
==26643==    by 0x4DCC43F: QMetaObject::activate(QObject*, int, int, void**) (qobject.cpp:3580)
==26643==    by 0x8905173: ImapClient::retrieveOperationCompleted() (imapclient.cpp:1735)
==26643==    by 0x895C707: ImapStrategyContextBase::operationCompleted() (imapstrategy.cpp:480)
==26643==    by 0x8962313: ImapRetrieveMessageListStrategy::messageListCompleted(ImapStrategyContextBase*) (imapstrategy.cpp:1345)