Bluetooth: Mesh: Disable prov bearers on link

Previously, PB-GATT unprovisioned advs would continue being sent if a
device is provisioned or in the process of being provisioned (an active
provisioning link) over PB-ADV.

This commit introduces the `link_cancel` callback for the `prov_bearer`
struct. When a provisioning link is established over a prov bearer, all
other (active) prov bearers will be suspended through this callback, and
re-enabled when the link is closed. An exception is made for PB-REMOTE,
which can utilize the other bearers.

The call to `bt_mesh_pb_gatt_srv_disable` in settings.c is removed. This
is called in `mesh_commit`, and is not needed as this is only used
during boot, at which point the Mesh provisioning service has not yet
been enabled/registered, meaning the function does nothing.

Signed-off-by: Håvard Reierstad <haavard.reierstad@nordicsemi.no>