quilt.inc: avoid using += with an over-ride (drop unmaintained darwin support)
Using += with an over-ride can be a source of confusion so try to
avoid the construct in core recipes.
In this case, the _darwin over-ride seems to be unmaintained. It was
added in early 2013 but has not been accounted for in subsequent
updates to the recipe and (from inspection) now appears to be broken.
Remove the _darwin over-ride rather than try to debug or fix it.
Signed-off-by: Andre McCurdy <armccurdy@gmail.com> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
