tests: Bluetooth: Mesh: Default to ext adv

Changes twister tests and nrf samples from using legacy adv to ext adv
since this is the default for Mesh. Legacy adv is still tested in bsim.

Removes redundant test configuration testing ext adv.

Removes redundant config for ext_adv in `multi_ext_adv.conf`.

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