UI: Fix source tree hovering being inconsistent

When hovering over the source tree items, sometimes the preview
would show the item hovered, sometimes not. This is caused by
the SourceTree mouseMoveEvent/leaveEvent calling the same functions
as the SourceTreeItem enterEvent/leaveEvent, therefore competing
with each other, causing some jankiness.

UI/source-tree.cpp

@@ -1082,8 +1082,6 @@ SourceTree::SourceTree(QWidget *parent_) : QListView(parent_)
 		"*[bgColor=\"7\"]{background-color:rgba(68,68,68,33%);}"
 		"*[bgColor=\"8\"]{background-color:rgba(255,255,255,33%);}"));
 
-	setMouseTracking(true);
-
 	UpdateNoSourcesMessage();
 	connect(App(), &OBSApp::StyleChanged, this,
 		&SourceTree::UpdateNoSourcesMessage);
@@ -1491,31 +1489,6 @@ void SourceTree::dropEvent(QDropEvent *event)
 	QListView::dropEvent(event);
 }
 
-void SourceTree::mouseMoveEvent(QMouseEvent *event)
-{
-	QPoint pos = event->pos();
-	SourceTreeItem *item = qobject_cast<SourceTreeItem *>(childAt(pos));
-
-	OBSBasicPreview *preview = OBSBasicPreview::Get();
-
-	QListView::mouseMoveEvent(event);
-
-	std::lock_guard<std::mutex> lock(preview->selectMutex);
-	preview->hoveredPreviewItems.clear();
-	if (item)
-		preview->hoveredPreviewItems.push_back(item->sceneitem);
-}
-
-void SourceTree::leaveEvent(QEvent *event)
-{
-	OBSBasicPreview *preview = OBSBasicPreview::Get();
-
-	QListView::leaveEvent(event);
-
-	std::lock_guard<std::mutex> lock(preview->selectMutex);
-	preview->hoveredPreviewItems.clear();
-}
-
 void SourceTree::selectionChanged(const QItemSelection &selected,
 				  const QItemSelection &deselected)
 {

UI/source-tree.hpp

@@ -211,8 +211,6 @@ public slots:
 protected:
 	virtual void mouseDoubleClickEvent(QMouseEvent *event) override;
 	virtual void dropEvent(QDropEvent *event) override;
-	virtual void mouseMoveEvent(QMouseEvent *event) override;
-	virtual void leaveEvent(QEvent *event) override;
 	virtual void paintEvent(QPaintEvent *event) override;
 
 	virtual void