staging: rtl8188eu: remove DBG_88E_LEVEL macro from include/rtw_debug.h

Remove definition of DBG_88E_LEVEL macro from include/rtw_debug.h, as it
no longer has any callers and does not follow kernel coding conventions.
Removing all such macros will ultimately allow the removal of this file.

Signed-off-by: Phillip Potter <phil@philpotter.co.uk>
Link: https://lore.kernel.org/r/20210623234902.7411-7-phil@philpotter.co.uk


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>