COMP: Support building DICOMWidget against Qt 5.12.8

Remove setting of unsupported and unused `placeholderText` property from
`ctkDICOMVisualBrowserWidget.ui`. Since the property is set to its default
value, removing it will not impact how the widget is rendered.

For reference, it was originally introduced in 88ff72b ("ENH: Add Visual
DICOM Browser (commontk#1165)", 2024-01-19)

Libs/DICOM/Widgets/Resources/UI/ctkDICOMVisualBrowserWidget.ui

@@ -403,9 +403,6 @@
                 <property name="sizeAdjustPolicy">
                  <enum>QComboBox::AdjustToContents</enum>
                 </property>
-                <property name="placeholderText">
-                 <string/>
-                </property>
                 <property name="frame">
                  <bool>true</bool>
                 </property>