create-spdx.bbclass: remove unused SPDX_INCLUDE_PACKAGED

[YOCTO #14948]

Signed-off-by: Michael Opdenacker <michael.opdenacker@bootlin.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
(cherry picked from commit 89f1abd5)
Signed-off-by: Steve Sakoman <steve@sakoman.com>