rtlwifi: rtl8723be: Remove set but not used variable 'b_last_is_cur_rdlstate'

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

drivers/net/wireless/realtek/rtlwifi/rtl8723be/dm.c: In function 'rtl8723be_dm_check_edca_turbo':
drivers/net/wireless/realtek/rtlwifi/rtl8723be/dm.c:998:7: warning:
 variable 'b_last_is_cur_rdlstate' set but not used [-Wunused-but-set-variable]

It's never used and can be removed.

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