[partition Simplify mountpoint-input

- if nothing is selected (index -1, which now shows the placeholder), the text is empty
- if something has been entered, return it (e.g. if the user is typing)
- if something is selected, the text gets set to that anyway

src/modules/partition/gui/PartitionDialogHelpers.cpp

@@ -55,10 +55,6 @@ standardMountPoints( QComboBox& combo, const QString& selected )
 QString
 selectedMountPoint( QComboBox& combo )
 {
-    if ( combo.currentIndex() == -1 )
-    {
-        return QString();
-    }
     return combo.currentText();
 }