classes/populate_sdk_base: remove nostamp from do_populate_sdk

classes/populate_sdk_base: remove nostamp from do_populate_sdk

We've removed nostamp from do_rootfs in image.bbclass in OE-Core commit
8505008c115efb54d18e5f25441c7a938a32ffaf, and we should do the same for
do_populate_sdk here for the same reason - we can now rely on task
signatures so if nothing has changed, we don't need to re-run it.

(From OE-Core rev: de9b693f4ff311f1310a1c6005e0d5c225aabef6)

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>