native: remove PN from DEPENDS automatically

If a recipe (say, wayland) has DEPENDS=wayland-native and BBCLASSEXTEND=native,
when built as wayland-native it has DEPENDS=wayland-native which results in a
circular dependency.

Typically this is resolved by having explicit DEPENDS_class-native statements
but as this is duplication which can lead to inconsistent dependencies,
automatically remove the circular dependency.

Signed-off-by: Ross Burton <ross.burton@intel.com>