rtl8xxxu: Remove set but not used variables 'usedesc40' and 'seq_number'

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

drivers/net/wireless/realtek/rtl8xxxu/rtl8xxxu_core.c: In function 'rtl8xxxu_tx':
drivers/net/wireless/realtek/rtl8xxxu/rtl8xxxu_core.c:4925:7: warning:
 variable 'usedesc40' set but not used [-Wunused-but-set-variable]

drivers/net/wireless/realtek/rtl8xxxu/rtl8xxxu_core.c:4921:6: warning:
 variable 'seq_number' set but not used [-Wunused-but-set-variable]

'usedesc40' and 'seq_number' are not used any more after
commit b59415c2 ("rtl8xxxu: Split filling of TX descriptors into separate functions")

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