staging: r8188eu: delete driver

Now that the same hardware that the r8188eu driver supported is
supported by the real wireless driver rtl8xxxu, the r8188eu driver can
be deleted.

Also the rtl8xxxu driver supports way more devices, and is a fraction of
the overall size, making this a much better overall solution.

Thanks to the r8188eu developers and maintainers and reviewers over the
years, your work allowed Linux users to use their hardware before the
real driver was implemented properly.

Reported-by: Hans de Goede <hdegoede@redhat.com>
Cc: Phillip Potter <phil@philpotter.co.uk>
Cc: Pavel Skripkin <paskripkin@gmail.com>
Acked-by: Larry Finger <LarryFinger@lwfinger.net>
Acked-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Martin Kaiser <martin@kaiser.cx>
Tested-by: Philipp Hortmann <philipp.g.hortmann@gmail.com> # Edimax N150
Acked-by: Michael Straube <straube.linux@gmail.com>
Link: https://lore.kernel.org/r/20230308131934.380395-1-gregkh@linuxfoundation.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>