staging: rtl8192u: Delete 'DM_CTSToSelfSetting' function in r8192U_dm.c

Delete the DM_CTSToSelfSetting function that is not used in
anywhere in the driver.

Fix sparse warnings:
drivers/staging/rtl8192u/r8192U_dm.c:2623:6: warning: symbol 'DM_CTSToSelfSetting' was not declared. Should it be static?

Signed-off-by: Ana Rey <anarey@gmail.com>
Acked-by: Luis R. Rodriguez <mcgrof@do-not-panic.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>