[Ash MD] Remove pre-MD code from TrayImageItem and subclasses

Remove the pre-MD code and now-unused PNG assets
from the TrayImageItem class, its subclasses,
and associated default rows in the Ash system menu.

BUG=686217,686256,687813,687814,687815,687818
TEST=manual, no functional changes

Review-Url: https://codereview.chromium.org/2700653002
Cr-Commit-Position: refs/heads/master@{#451392}
41 files changed