Remove redundant Gtk::Entry calls in Create New Partition dialog

This is a small tidy-up to remove Gtk::Entry method calls on the file
system label entry box in the Create New Partition dialog which serve no
purpose.

filesystem_label_entry.set_activates_default( true );
    It trying to make the Create New Partition dialog automatically
    close  when Enter is pressed with focus in the label entry box.
    However this doesn't work, presumably because the default widget for
    the dialog is not the Add button.  Remove.

filesystem_label_entry.set_text( partition.get_filesystem_label() );
    Initialises the text in the entry box with the file system label
    from the passed partition object.  The label is blank and the entry
    box defaults to blank.  Achieves nothing.  Remove.

filesystem_label_entry.select_region( 0, filesystem_label_entry.get_text_length() );
    Highlights the empty text in the entry box.  Achieves nothing.
    Remove.

NOTE:
The same set of Gtk::Entry method calls in Dialog_FileSystem_Label() and
Dialog_Partition_Name, which are editing the existing file system label
and partition name respectively, do work and have a useful effect so
shouldn't be removed.