mac80211: remove STA txq pending airtime underflow warning

mac80211: remove STA txq pending airtime underflow warning

This warning can trigger if there is a mismatch between frames that were
sent with the sta pointer set vs tx status frames reported for the sta address.

This can happen due to race conditions on re-creating stations, or even
in the case of .sta_add/remove being used instead of .sta_state, which can cause
frames to be sent to a station that has not been uploaded yet.

If there is an actual underflow issue, it should show up in the device airtime
warning below, so it is better to remove this one.

Signed-off-by: Felix Fietkau <nbd@nbd.name>
Link: https://lore.kernel.org/r/20200725084533.13829-1-nbd@nbd.name
Signed-off-by: Johannes Berg <johannes.berg@intel.com>