[nrf fromlist] Bluetooth: Mesh: fix PrivateBeaconKey PSA key leak on subnet delete

subnet_keys_destroy() guarded the destroy of keys->priv_beacon with
matching import with #if defined(CONFIG_BT_MESH_PRIV_BEACONS).

Commit 981c79b ("Bluetooth: Mesh: Drop explicit support for
Bluetooth Mesh 1.0.1") removed the CONFIG_BT_MESH_V1d1 Kconfig but
left this stray reference behind. With V1d1 gone, the destroy branch
is permanently dead code, so every successful net_keys_create() leaks
one PSA key slot.

In a long-running test that repeatedly provisions and resets a node
(no persistent settings, in-RAM CDB only), the leak accumulates one
PSA volatile-key slot per provisioning round. With the default
CONFIG_MBEDTLS_PSA_KEY_SLOT_COUNT=16, bt_mesh_private_beacon_key()
fails after ~12 rounds with:

  <err> bt_mesh_net_keys: Unable to generate private beacon key
  <err> bt_mesh_net: Failed creating subnet
  <err> bt_mesh_main: Failed to create network

Align the destroy guard with the import guard so the leak goes away.

Upstream PR #: 110235

Signed-off-by: Lingao Meng <menglingao@xiaomi.com>

Author: LingaoM

subsys/bluetooth/mesh/subnet.c

@@ -202,7 +202,7 @@ static void subnet_keys_destroy(struct bt_mesh_subnet_keys *key)
 #if defined(CONFIG_BT_MESH_GATT_PROXY)
 	bt_mesh_key_destroy(&key->identity);
 #endif
-#if defined(CONFIG_BT_MESH_V1d1)
+#if defined(CONFIG_BT_MESH_PRIV_BEACONS)
 	bt_mesh_key_destroy(&key->priv_beacon);
 #endif
 }