UI: Fix maximum size on image source toolbar

The edit widget on the image source toolbar had a maximum width set,
which caused the layout for the toolbar to look ugly if it hit that
maximum width.

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

@@ -50,12 +50,6 @@
        <height>0</height>
       </size>
      </property>
-     <property name="maximumSize">
-      <size>
-       <width>600</width>
-       <height>16777215</height>
-      </size>
-     </property>
      <property name="text">
       <string notr="true"/>
      </property>