net80211: fix TIM cleanup.

Remove duplicate 'ni->ni_associd = 0' assignment from
ieee80211_node_leave(), since it breaks iv_set_tim() in
ic->ic_node_cleanup() (associd is cleared right after this call).

Tested with RTL8188EU (HOSTAP mode) and
WUSB54GC (STA mode, with powersaving enabled).

Approved by: adrian (mentor)
Differential Revision: https://reviews.freebsd.org/D5398