wifi: rtw88: remove unused and set but unused leftovers

Drop unused and set but unused 'last_push' of 'struct rtw_txq',
'wireless_set' of 'struct rtw_sta_info', 'usb_txagg_num' of
'struct rtw_usb' and 'n' of 'struct rx_usb_ctrl_block', unused
definition of 'struct rtw_timer_list', adjust related code.

Reviewed-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Dmitry Antipov <dmantipov@yandex.ru>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20230628072327.167196-2-dmantipov@yandex.ru