rtlwifi: rtl8192cu: remove set but not used variable 'turbo_scanoff'

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/net/wireless/realtek/rtlwifi/rtl8192cu/rf.c: In function 'rtl92cu_phy_rf6052_set_cck_txpower':
drivers/net/wireless/realtek/rtlwifi/rtl8192cu/rf.c:45:7: warning: variable 'turbo_scanoff' set but not used [-Wunused-but-set-variable]

It is not used any more since
commit e9b0784b

 ("rtlwifi: rtl8192cu: Fix some code in RF handling")

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>