A minor performance optmisation to keep lists smaller when running large
builds. We can do this since once a task has been built, we don't need
to worry about it. This improves a major bottleneck that shows up on
performance profile charts in dryruns.
(Bitbake rev: cd6b89230823707c3c9bb9e6883bf5a971916581)
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>