rtw88: remove unused variable 'in_lps'

Unused, will not be used neither, because the hardware/firmware
can only support one vif for LPS currnetly. If there's more than
one vif, than driver will never enter LPS. So remove it.

Signed-off-by: Yan-Hsuan Chuang <yhchuang@realtek.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>