rm_work: exclude the SSTATETASKS from the rm_work tasks sinature

We can exclude the SSTATETASKS from the rm_work task signature
to avoid running the task when we remove some setscene tasks
from the dependencie chain.

The inject_rm_work handler on the rm_work bbclass triggers the
rm_work task running for any signature change in the dependencie
chain of the task do_build of each recipe.

i.e INHERIT:remove = "create-spdx" will trigger the do_rm_work
when we collect the sstate cache with INHERIT = "create-spdx"

Signed-off-by: Jose Quaresma <jose.quaresma@foundries.io>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>