UI: Remove spacing from scene and source tree

Due to QTBUG-106395, sources or scenes that are drag-and-dropped into
the spacing between other sources and scenes would currently go to the
bottom of the list, leaving users confused. Until that is fixed, we have
to remove this spacing.
The call in scene-tree.cpp was redundant anyways, should this commit get
reverted in the future it doesn't need to be re-added.

UI/forms/OBSBasic.ui

@@ -794,7 +794,7 @@
            </sizepolicy>
           </property>
           <property name="spacing">
-           <number>1</number>
+           <number>0</number>
           </property>
           <property name="contextMenuPolicy">
            <enum>Qt::CustomContextMenu</enum>
@@ -932,7 +932,7 @@
            </sizepolicy>
           </property>
           <property name="spacing">
-           <number>1</number>
+           <number>0</number>
           </property>
           <property name="contextMenuPolicy">
            <enum>Qt::CustomContextMenu</enum>

UI/scene-tree.cpp

@@ -91,7 +91,6 @@ void SceneTree::resizeEvent(QResizeEvent *event)
 		}
 	} else {
 		setGridSize(QSize());
-		setSpacing(1);
 		for (int i = 0; i < count(); i++) {
 			item(i)->setData(Qt::SizeHintRole, QVariant());
 		}