Remove the (unused) valid bit from trajectories.

[jeremykubica]

The valid bit was meant to be a way to indicate that something is filtered on the GPU, but we use num_obs or likelihood instead. We can drop this bit to save both space in the results and time in filtering without changing behavior.

Also does a few smaller cleanups:

• Adds some more logging and timing
• Removes unused sorting and filtering code from the TrajectoryList.