staging: r8188eu: remove last_tx_complete_time

last_tx_complete_time from struct sreset_priv is set / calculated when a
packet is sent out. Like other sreset_priv components, it is not read and
can be removed.

Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Link: https://lore.kernel.org/r/20211024180448.20624-5-martin@kaiser.cx
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>