nrf51822: Remove app_error_handler() function

This function was needed to handle errors from Nordic SDK via
APP_ERROR_CHECK/APP_ERROR_HANDLER macros.
stack: Add delay API
nrf51822: Implement delay module
nrf51822/log: Replace nrf_delay module by delay
examples/ll-scanner: Replace nrf_delay module by delay
nrf51822: Remove nrf_delay.c from sources list