staging: rtlwifi: rtl8822be: Remove set but not used variable 'curtxbw_40mhz'

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

drivers/staging/rtlwifi/rtl8822be/hw.c: In function 'rtl8822be_update_hal_rate_mask':
drivers/staging/rtlwifi/rtl8822be/hw.c:2144:5: warning:
 variable 'curtxbw_40mhz' set but not used [-Wunused-but-set-variable]

It's never used and can be removed.

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Reviewed-by: Mukesh Ojha <mojha@codeaurora.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>