native: Use deltask to remove tasks which have no purpose

core-image-sato has 47703 inter task dependencies before this patch
and 29883 afterwards which is a significant worthwhile task graph
simplification.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
externalsrc: Use deltask API

Now we have deltask API, stop poking around bitbake internal variables.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>