Use base class's 'Qt5FilePicker::getDirectory' instead
to reduce duplication.

Strictly speaking, the 'KDE5FilePicker::implGetDirectory'
implementation used by 'KDE5FilePicker::getDirectory'
suggests that the display directory is returned
rather than the selected directory, and the same
method 'implGetDirectory()' was actually previously used
by the 'KDE5FilePicker::getDisplayDirectory()' method
as well (removed in a previous commit).

The code worked fine inside KDE5FilePicker (where the
native Plasma/kde5 file dialog is used) but would lead
to incorrect results when used inside Qt5FilePicker, e.g.
for the following scenario

* open  folder picker displaying $HOME that contains
  a directory "bar"
* do a single mouse click on the "bar" directory
* click "OK"

The call to 'toOUString(m_pFileDialog->directoryUrl().url())'
inside 'getDirectory()' would return '$HOME/bar' for the native
QFileDialog on KDE Plasma 5, but '$HOME' when
used with the non-native QFileDialog

Anyway, the implementation inside 'Qt5FilePicker::getDirectory'
works fine for both cases, so just drop the KDE5FilePicker one.

Change-Id: I86dcf98ed310636b899ad289d8a8a8fa263dd2e9
Reviewed-on: https://gerrit.libreoffice.org/68050
Tested-by: Jenkins
Reviewed-by: Katarina Behrens <Katarina.Behrens@cib.de>