UI: Make image source toolbar expand

The entire image source file couldn't be seen in some cases, so just
make it take up the full width

UI/forms/source-toolbar/image-source-toolbar.ui

@@ -77,22 +77,6 @@
      </property>
     </widget>
    </item>
-   <item>
-    <spacer name="horizontalSpacer">
-     <property name="orientation">
-      <enum>Qt::Horizontal</enum>
-     </property>
-     <property name="sizeType">
-      <enum>QSizePolicy::Expanding</enum>
-     </property>
-     <property name="sizeHint" stdset="0">
-      <size>
-       <width>40</width>
-       <height>20</height>
-      </size>
-     </property>
-    </spacer>
-   </item>
   </layout>
  </widget>
  <resources/>