rtlwifi: rtl8192ee: Remove set but not used variable 'cur_tx_wp'

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

drivers/net/wireless/realtek/rtlwifi/rtl8192ee/trx.c: In function rtl92ee_is_tx_desc_closed:
drivers/net/wireless/realtek/rtlwifi/rtl8192ee/trx.c:1005:18: warning: variable cur_tx_wp set but not used [-Wunused-but-set-variable]

It is not used since commit cf54622c ("rtlwifi:
cleanup the code that check whether TX ring is available")

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