classes/sstate: fix taints being undone on execution of sstate tasks
The code here that deletes stamps was also deleting the taint files; so
forcing an sstate task with -f would force it to execute and then
because the taint file was deleted in the process, the next execution
would simply restore the output from sstate again. We need to exclude
the taint files just like we did in bb.build.make_stamp().
Fixes [YOCTO #5805].
Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
