staging: rtl8188eu: Remove function rtw_setdatarate_cmd()

rtw_setdatarate_cmd() enqueue _SetDataRate command but there is not
handler mapped in wlancmds[] to handle this command.

Signed-off-by: navin patidar <navin.patidar@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>