sstate: Ensure deploy tasks don't pull in toolchains

If an image is built from sstate, the cross toolchain was being pulled in.
This was due to the sstate dependencies for deploy tasks not being considered
in the "depvalid" logic.

do_deploy tasks do not need their populate_sysroot dependencies when installed
from sstate so skip these. This reduces the sstate pulled in by an image
build from sstate significantly.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>