Remove RackView widget before deleting the ChannelView

Fixes the following two errors I spotted using valgrind:
When deleting a channel;
==936== Invalid read of size 8
==936==    at 0x56FA1D: FxMixerView::deleteChannel(int) (FxMixerView.cpp:374)
==936==    by 0x60E9A79: QMetaObject::activate(QObject*, QMetaObject const*, int, void**) (in /usr/lib/x86_64-linux-gnu/libQtCore.so.4.8.6)
==936==    by 0x5216BF1: QAction::triggered(bool) (in /usr/lib/x86_64-linux-gnu/libQtGui.so.4.8.6)
==936==    by 0x52185C2: QAction::activate(QAction::ActionEvent) (in /usr/lib/x86_64-linux-gnu/libQtGui.so.4.8.6)
==936==  Address 0x14d51b90 is 32 bytes inside a block of size 40 free'd
==936==    at 0x4C2C2E0: operator delete(void*) (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==936==    by 0x56F9ED: FxMixerView::deleteChannel(int) (FxMixerView.cpp:370)
==936==    by 0x60E9A79: QMetaObject::activate(QObject*, QMetaObject const*, int, void**) (in /usr/lib/x86_64-linux-gnu/libQtCore.so.4.8.6)
==936==    by 0x5216BF1: QAction::triggered(bool) (in /usr/lib/x86_64-linux-gnu/libQtGui.so.4.8.6)

When loading a new project after adding some channels:
==936== Invalid read of size 8
==936==    at 0x570785: FxMixerView::refreshDisplay() (FxMixerView.cpp:202)
==936==    by 0x4B590E: Song::clearProject() (Song.cpp:740)
==936==    by 0x4B7885: Song::createNewProject() (Song.cpp:817)
==936==    by 0x60E9A79: QMetaObject::activate(QObject*, QMetaObject const*, int, void**) (in /usr/lib/x86_64-linux-gnu/libQtCore.so.4.8.6)
==936==  Address 0x56a12ab0 is 32 bytes inside a block of size 40 free'd
==936==    at 0x4C2C2E0: operator delete(void*) (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==936==    by 0x57075B: FxMixerView::refreshDisplay() (FxMixerView.cpp:201)
==936==    by 0x4B590E: Song::clearProject() (Song.cpp:740)
==936==    by 0x4B7885: Song::createNewProject() (Song.cpp:817)