When a single signature is locked, dependent task checksum calculations
fail. This in turn is because get_unihash cannot be cached correctly
by this function. Remove that has turned out to be a poor optimisation
to avoid that bug.
(Bitbake rev: 7ae2b73d131491ab5bf09fd6055e1fdb67a560c6)
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>