staging: rtl8192u: remove unused Makefile

drivers/staging/rtl8192u/ieee80211/Makefile is not used at all.
All the build rules are described in drivers/staging/rtl8192u/Makefile.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Link: https://lore.kernel.org/r/20200104162136.19170-1-masahiroy@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>