libchromeos: remove libchromeos-ui.

libchromeos-ui is moving back to src/platform2 as it is only used on
chromeos.

BUG=chromium:525211
TEST=manual: `emerge-gizmo libchromeos` does not install libchromeos-ui.
TEST=trybots.

Change-Id: Id16af4b9f4aa090ef2c9805f6e963773bd62a881
10 files changed