tests: Bluetooth: Mesh: Remove device synchronization API

This API was creating more problems than solving. We should not use back
channel in bsim tests unless there is no other way to communicate
between devices. To synchronize devices we should use k_sleep.

This addresses
https://github.com/zephyrproject-rtos/zephyr/issues/55821



Signed-off-by: Pavel Vasilyev <pavel.vasilyev@nordicsemi.no>