rtlwifi: btcoex: Remove set but not used variable 'result'

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

drivers/net/wireless/realtek/rtlwifi/btcoexist/halbtc8192e2ant.c: In function btc8192e2ant_tdma_duration_adjust:
drivers/net/wireless/realtek/rtlwifi/btcoexist/halbtc8192e2ant.c:1584:6: warning: variable result set but not used [-Wunused-but-set-variable]

It is not used since commit 27a31a60 ("rtlwifi:
btcoex: remove unused functions")

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: zhengbin <zhengbin13@huawei.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>