sstate: Avoid deploy_source_date_epoch sstate when unneeded

This sstate task is only needed when depended upon, it can be skipped
if there are no tasks running that directly depend upon it.

This reduced the number of sstate tasks in something like an image
build.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 601cee01)
Signed-off-by: Steve Sakoman <steve@sakoman.com>