qt: remove useless simple prefs layout spacer

This was placed into the same grid row+column as a spinbox and thus
achieved absolutely nothing. Presumably it was intended for the adjacent
column, however since it achieves nothing either way let's just drop it.

modules/gui/qt/dialogs/preferences/sprefs_input.ui

@@ -37,19 +37,6 @@
         </property>
        </widget>
       </item>
-      <item row="6" column="1">
-       <spacer name="horizontalSpacer">
-        <property name="orientation">
-         <enum>Qt::Horizontal</enum>
-        </property>
-        <property name="sizeHint" stdset="0">
-         <size>
-          <width>40</width>
-          <height>20</height>
-         </size>
-        </property>
-       </spacer>
-      </item>
       <item row="4" column="0">
        <widget class="QLabel" name="hwAccelLabel">
         <property name="text">