Eliminate misleading warning.

Remove token from deferred list of tokens to be removed once token
is removed. Leaving it in the list leads to logging messages like
"WindowManager: removeAppFromTaskLocked: token=AppWindowToken{...
} not found" when an attempt to remove it a second time fails.

Discovered in logs from b/17512377.

Change-Id: Ic83d81841b9b74ae5c4c433d1086d3bbda8e1d64