staging: rtl8192u: remove entry from Makefile

staging: rtl8192u: remove entry from Makefile

In commit 697455ce4110 ("staging: rtl8192u: Remove broken driver"), the
driver was removed, along with the Kconfig entry, but the Makefile line
in drivers/staging/Makefile was not updated, so things like 'make clean'
fail to work properly as they will decend into all subdirectories to try
to clean things up.

Resolve this by removing the entry in the main staging Makefile.

Reported-by: Thorsten Leemhuis <linux@leemhuis.info>
Cc: Philipp Hortmann <philipp.g.hortmann@gmail.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Fixes: 697455ce4110 ("staging: rtl8192u: Remove broken driver")
Tested-by: Thorsten Leemhuis <linux@leemhuis.info>
Link: https://lore.kernel.org/r/2023101709-amuck-upward-46f1@gregkh
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>