Bluetooth: Mesh: Keep PB-GATT callback through disconnect

Removes the memset of the prov_bearer_cb in PB-GATT during resets. This
allows the provisioning link to disconnect and reconnect again without
having to call pb_gatt_open.

Fixes #26343.

Signed-off-by: Trond Einar Snekvik <Trond.Einar.Snekvik@nordicsemi.no>