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

drivers/net/wireless/realtek/rtlwifi/rtl8723ae/hal_btc.c: In function '_rtl8723e_dm_bt_coexist_2_ant':
drivers/net/wireless/realtek/rtlwifi/rtl8723ae/hal_btc.c:1408:5: warning:
 variable 'bt_retry_cnt' 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>