staging: rtl8188eu: remove rtl8188eu driver from staging dir

This driver was deprecated with the introduction of the r8188eu driver,
based upon Realtek sources that were modified for CFG80211 support and
other fixes on GitHub by Larry Finger. As that driver is now progressing
at pace, we should remove this one.

Signed-off-by: Phillip Potter <phil@philpotter.co.uk>
Link: https://lore.kernel.org/r/20210731133809.196681-1-phil@philpotter.co.uk
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>