HS 2.0: Remove duplicate icon entries

HS 2.0: Remove duplicate icon entries

Only one of the icon entries with a matching BSSID and file name can be
fetched from wpa_supplicant and as such, there is no need to maintain
the old data if it was not explicitly deleted before running a new fetch
for the same BSSID and icon. Remove older duplicated entries whenever
completing a pending icon fetch to optimize memory use.

Signed-off-by: Jouni Malinen <j@w1.fi>