externalsrc: fix dependency chain issues

Instead of deleting setscene tasks, now SSTATE_SKIP_CREATION is set instead.

This seems to fix the compile issues where the populate_sysroot task was
not run when an externalsrc recipe was built as a dependency.

[YOCTO #15164]

[RP addition: The deltask was added by me in 2012 when the class was created.
The trouble is bitbake assumes 'sstate' tasks have a setscene task and by deleting
the setscene task, bitbake stops thinking the task can be accelerated. There is other
code in the sysroot code which assumes some tasks are always sstate tasks.

We cannot delete the task without changes to the way bitbake learns about 'setscene'
tasks so the patch is correct, avoiding creating files is the better approach given
the way the world works now.

There would be concerns about exisitng sstate reuse however this shouldn't occur
since SRC_URI changes and that will change the underlying hashes. Hash equivalency
could potentially cause issues by joining hashes together again however if the output
matches, that shouldn't in theory cause any issue.]

Signed-off-by: Peter Suti <peter.suti@streamunlimited.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>