Bluetooth: Host: Fix compiling PAwR Sync without PAST RX

The function `bt_hci_le_past_received_v2()` is not compiled
in for this configuration, so the reference needs to be removed.

Fixes #76268.

Signed-off-by: Rubin Gerritsen <rubin.gerritsen@nordicsemi.no>
(cherry picked from commit e23345b4)