scripts: edtlib: do not inherit parent compatibles

Commit 0d4dca10 ("scripts: edtlib:
child binding compatibles match parents") was a hack meant to keep the
edtlib.Binding class in place without modifying some twister behavior
that needed further changes to work properly with first-class binding
objects.

This is a hack and is no longer necessary, so back out of this change.

Child Binding objects now have None compatible properties unless the
binding YAML explicitly sets a compatible.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>