Remove unused planner config params.

- The defaults for the longest_valid_segment_fraction are good enough.
  I played with custom fixed values to see if I could improve plan quality,
  but only increased computation times, and a marginal quality improvement.

- Removing the projection_evaluator makes moveit default to RRT-connect, which
  is faster and yields better plans than Kpiece .I still need to understand
  how default planners are chosen.
Remove head_mount_xtion_link from collision ops.

- This was causing a lot of (inocuous) error log statements on MoveIt! startup.

- This link is not part of the default robot, but when the setup assistant was
  ran it was there, hence MoveIt! complained about links present in the SRDF
  but not in the URDF.