scripts/dts: Remove dead and confusing code in get_phandles()

  - The 'props' key is known to exist (and 'if enabled == ...:' would
    crash if it ever was missing)

  - 'd' is always a dictionary

  - There's no need to test if d['children'] is non-empty before looping
    through it

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>