UI: Only use volume scrollbars when needed

The scrollbars were always set to be on, no matter
how many volume controls that were being used.

UI/forms/OBSBasic.ui

@@ -1016,7 +1016,7 @@
          <enum>QFrame::Sunken</enum>
         </property>
         <property name="verticalScrollBarPolicy">
-         <enum>Qt::ScrollBarAlwaysOn</enum>
+         <enum>Qt::ScrollBarAsNeeded</enum>
         </property>
         <property name="horizontalScrollBarPolicy">
          <enum>Qt::ScrollBarAlwaysOff</enum>
@@ -1072,7 +1072,7 @@
          <enum>Qt::ScrollBarAlwaysOff</enum>
         </property>
         <property name="horizontalScrollBarPolicy">
-         <enum>Qt::ScrollBarAlwaysOn</enum>
+         <enum>Qt::ScrollBarAsNeeded</enum>
         </property>
         <property name="widgetResizable">
          <bool>true</bool>