sstate: Drop 'SafeDep' code from setscene validation function

I have a feeling this code exists from the time before we had proper
coverage of one sstate task by another task. At that time it was a
"poor" persons version of that idea, we now have much better
code internal to bitbake which handles this.

Worse, this code actually breaks certain rebuild scenarios,
e.g.:

bitbake libtool-cross
bitbake libtool-cross -c cleansstate
rm tmp -rf
bitbake libtool-cross

would fail as binutils-cross wasn't installed from sstate.

The easiest fix is to remove the obsolete/broken code.

[YOCTO #5773]

(From OE-Core master rev: ccad07f35fb7f959e24fd50d04c7d10dd5cf20d0)

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