Bluetooth: host: fix conditional compile for broadcaster role

The call to bt_adv_reset_adv_pool is only required in the
broadcaster role, and not in the observer role, regardless
of the setting of CONFIG_BT_EXT_ADV
As the code was this call was also made for the observer.

Note: handling of the setting of CONFIG_BT_EXT_ADV
is already handled in the bt_adv_reset_adv_pool function

Signed-off-by: Andries Kruithof <andries.kruithof@nordicsemi.no>