Fix crash on agent_launcher exit.

Don't unload plugins here. They will be unloaded on application exit,
so it's redundant.

Seems they are being unloaded too early, before QThreadStorage
cleanup occurs:

==18802== Invalid read of size 8
==18802==    at 0x53178AE: QThreadStorage<QFontCache*>::deleteData(void*) (in
/data/installation/qt/qt-4.7/lib/libQtGui.so.4.7.1)
==18802==    by 0x5E8F8CF: QThreadStorageData::set(void*)
(qthreadstorage.cpp:165)
==18802==    by 0x5317940: void
qThreadStorage_setLocalData<QFontCache>(QThreadStorageData&, QFontCache**)
(qthreadstorage.h:92)
==18802==    by 0x53165BA:
QThreadStorage<QFontCache*>::setLocalData(QFontCache*) (qthreadstorage.h:148)
==18802==    by 0x531407A: QFontCache::cleanup() (qfont.cpp:2632)
==18802==    by 0x5406536: QFont::cleanup() (qfont_x11.cpp:182)
==18802==    by 0x5104E35: qt_cleanup() (qapplication_x11.cpp:2647)
==18802==    by 0x50603B2: QApplication::~QApplication()
(qapplication.cpp:1172)
==18802==    by 0x41B1F6: main (agentlauncher.cpp:28)
==18802==  Address 0x11a090b0 is not stack'd, malloc'd or (recently) free'd

CCBUG: 261788