staging: rtl8188eu: remove unimplemented functions declaration

These functions have been declared without any implementation since
the first commit (58c43401) and there
has been no mention of them in following commits.

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Cc: Kyle Kuffermann <kyle.kuffermann@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>