Remove directory tree menu item

This is dead code, since "Add service" migrated to "gear menu"
crrev.com/c/1139944.

Remove:
1. ADD_NEW_SERVICES_MENU constant from VolumeManagerCommon.RootType.
2. Style and icons specific for displaying "Add service" in the
directory tree.
3. Icons that was used for "Add service" in directory tree.
4. NavigationModelMenuItem and MenuItem types.

NOTE: Translation strings are still in use in the gear menu so we won't
remove them.

Bug: 869252
Change-Id: Ic6c0abf74dff6cf2ebf2821b88d930dcd1653bbf
Reviewed-on: https://chromium-review.googlesource.com/c/1260882
Reviewed-by: Joel Hockey <joelhockey@chromium.org>
Commit-Queue: Luciano Pacheco <lucmult@chromium.org>
Cr-Commit-Position: refs/heads/master@{#596950}
8 files changed