Bluetooth: Mesh: remove 20ms tx delay in adv bearer

The recommendation to have 20ms is fair for two
consecutive messages over a single bearer. When mesh
sends two messages it can be interpreted as two
bearers working in parallel. No need to keep
an artificial 20ms delay for that. Delay was
removed and all related bsim tests were fixed.

Signed-off-by: Aleksandr Khromykh <aleksandr.khromykh@nordicsemi.no>